Unfashionable affair, better (5)
I believe the answer is:
outdo
'better' is the definition.
(thesaurus)
'unfashionable affair' is the wordplay.
'unfashionable' becomes 'out' ('out' can be a synonym of 'unfashionable').
'affair' becomes 'do' (do is a kind of affair).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Perform better than, be superior to" , "Perform better than (someone)" , "Eclipse" , "Get the better of" , "Defeat" .)