Fashionable to allow vent (5)
I believe the answer is:
inlet
'vent' is the definition.
(thesaurus)
'fashionable to allow' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'to allow' becomes 'let' ('let' can be a synonym of 'allow').
'in'+'let'='INLET'
(Other definitions for inlet that I've seen before include "access point" , "Bay, bight" , "One's up the creek here" , "Small arm of the sea" , "Arm of lake, river" .)