Cohabiting, as it happens, fashionable (4,2)
I believe the answer is:
live in
'cohabiting' is the definition.
(I've seen this before)
'as it happens fashionable' is the wordplay.
'as it happens' becomes 'live' (as in live television).
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'live'+'in'='LIVE-IN'
(Other definitions for live in that I've seen before include "Resident at one's place of work" , "(Of employee or lover) resident" , "Cohabiting" , "Reside at one's place of employment" .)