Lover to live with a Unionist (4)
I believe the answer is:
beau
'lover' is the definition.
(beau is a kind of lover)
'live with a unionist' is the wordplay.
'live' becomes 'be' ('be' can be a synonym of 'live').
'with' means one lot of letters go next to another.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'be'+'a'+'u'='BEAU'
'to' is the link.
(Other definitions for beau that I've seen before include "An old boy friend like Brummel?" , "Lover boy" , "young man" , "One loving" , "Admirer; fop, dandy" .)