Alison I fancy for an affair (7)
I believe the answer is:
liaison
'an affair' is the definition.
('liaison' can be a synonym of 'affair')
'alison i fancy' is the wordplay.
'fancy' is an anagram indicator.
'alison'+'i'='alisoni'
'alisoni' with letters rearranged gives 'LIAISON'.
'for' acts as a link.
(Other definitions for liaison that I've seen before include "Connection - relationship" , "Channel of communication" , "An affair or mediation" , "A contact or affair" , "Secretive affair" .)