Live with a posh lover (4)
I believe the answer is:
beau
'lover' is the definition.
(I know that beau is a type of lover)
'live with a posh' is the wordplay.
'live' becomes 'be' (synonyms).
'with' says to put letters next to each other.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'be'+'a'+'u'='BEAU'
(Other definitions for beau that I've seen before include "Dandy - attentive companion" , "Fop; lover" , "Lover boy" , "One loving" , "Dandy - lover" .)