Bay allowed to trail in (5)
I believe the answer is:
inlet
'bay' is the definition.
(thesaurus)
'allowed to trail in' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to trail' is a charade indicator (letters next to each other).
'let' put after 'in' is 'INLET'.
(Other definitions for inlet that I've seen before include "Entry; arm of the sea" , "Coastal bay" , "Fjord" , "access point" , "Small arm of the sea" .)