Adult is trapped in feline affair (7)
I believe the answer is:
liaison
'affair' is the definition.
(synonyms)
'adult is trapped in feline' is the wordplay.
'adult' becomes 'a' (abbreviation).
'trapped in' is an insertion indicator.
'feline' becomes 'lion' (lion is a kind of feline).
'a'+'is'='ais'
'ais' inserted inside 'lion' is 'LIAISON'.
(Other definitions for liaison that I've seen before include "An affair or mediation" , "Intimate relationship" , "Communication between organisations - or an illicit affair" , "Amour; communication" , "Channel of communication" .)