Make love in retreat, mostly with one aristocrat (4,2,3)

'make love' is the definition.
(synonyms)

'retreat mostly with one aristocrat' is the wordplay.
'retreat' becomes 'haven' (retreat can mean a sanctuary or haven).
'mostly' means to remove the last letter.
'with one' becomes 'i' (Roman numeral. I am not sure about the 'with' bit.).
'aristocrat' becomes 'toff' (toff can mean an upper-class person).
'haven' with its final letter removed is 'have'.
'have'+'i'+'toff'='HAVE IT OFF'

'in' is the link.