The phrase in clover means in ease and luxury. It refers to the use of clover as fodder—as explained by the English lexicographer Samuel Johnson (1709-1784) in A Dictionary of the English Language: In which the Words are deduced from their Originals, and Illustrated in their Different Significations by Examples from the best Writers. To which are Prefixed, a History of the Language, and an English Grammar (London: Printed by W. Strahan, For J. and P. Knapton; T. and T. Longman; C. Hitch and L. Hawes; A. Millar; and R. and J. Dodsley – 1755):

To live in Clover, is to live luxuriously; clover being extremely delicious and fattening to cattle.

These are the earliest occurrences of the phrase that I have found, in chronological order:

1-: From Poems. To the Divine Apollo, published in The British Apollo: or, Curious Amusements for the Ingenious. To which are added the most Material Occurrences Foreign and Domestick. From Wednesday February 22d. to Friday February 25th. 1710 (London: Printed for, and Sold by J. Mayo, at the Printing-Press, over against Water-Lane in Fleet-street)—the slang noun rino denotes money:

I liv’d in Clover, to my thinking,
’Till I perceiv’d the Rino sinking.

2-: From The Caledonian Mercury (Edinburgh, Midlothian, Scotland) of Tuesday 21^st May 1734:

The last Letters from the Camp of Germany are of the 15th […]. The Number in that Camp is 75 Battalions and 105 Squadrons, who are allowed to live at Discretion, that is the Soldiers are permitted to go out a maroding on their private Account, and pillage or destroy at Will, whilst a regular Body of 8 Battalions and 13 Squadrons, under Lieut. General Quodt, put the Country of Wirtemburg under Contribution. This Liberty has produced great Abundance; Cows, Hogs and other Cattle, are brought daily into the Camp, with Fruits, Wine and other Things: The Soldiers live in Clover, the Officers grow rich and the Country People Wise, if Affliction can make them so.

3-: From the beginning of The Miser’s Feast. The Eighth Satire of the Second Book of Horace * Imitated. A Dialogue between the Author and the Poet Laureat (London: Printed for R. Dodsley, at Tully’s Head, Pall Mall, 1737), by the British translator George Ogle (baptised 1704 – died 1746):

Well, ¹ Laureat, was the day in clover spent?
How far’d you with the Priest? the Priest
A chearful friend, ³ desirous to invite,
I heard you ⁴ was engag’d for noon and night.
Plenty of meat and wine, ⁵ no doubt, you found,
Now he has made ² a hundred thousand pound.

Sat. VIII. Lib. II.
Horatius & Fundanius.

Hor. Ut Nasidieni juvit te cœna ¹
Beati ? ²
Nam mihi convivam quærenti, ³
Dictus here illic ⁴
De medio potare die. ⁵

(* Horace (Quintus Horatius Flaccus – 65-8 BC) was a Roman poet of the Augustan period.)

Horace’s Latin text is as follows in Q. Horatii Flacci Poëmata. The Works of Horace (London: Printed for Thomas Tegg and Son, 1837), edited by Charles Anthon and James Boyd:

Ut Nasidieni juvit te cœna beati ?
Nam mihi convivam quærenti, dictus heri illic
De medio potare die.

And this Latin text translates as follows:

How was dinner with Nasidienus the blessed?
For I was told yesterday, when seeking to make you my guest,
That you were drinking there since noon.

4-: From P-rs-n G—lb—rt’s address to the Bishops and Clergy, versified, published in The Scots Magazine (Edinburgh, Midlothian, Scotland) of December 1745:

It is well known all the world over,
The English clergy live in clover;
That they have all things at command,
Money good store, as well as land.

5-: From A Dictionary of the English Language (1755), by Samuel Johnson—as quoted above.

6-: From Adams’s Weekly Courant (Chester, Cheshire, England) of Tuesday 12^th January 1768:

To the PRINTER, &c.

HARD as the Times are for the Poor in many Places, we in this Borough, thank God! never eat or drank better in all our Days than we do at present; and all that we wish, is, that the same good-living may continue: and as the Parliament are considering of proper Methods to make Provisions plenty, we poor Folks in this Town are all of Opinion, that nothing the Parliament can do, would make Provisions so plenty amongst us as their making a Law, that Parliaments should be chosen every Year, instead of every seven; for then we should always live in Clover, and never want a good Belly-full.
We are your’s [sic], &c.
Two Hundred Pot-wallopers.