Sea inlet (7)
I believe the answer is:
estuary
'sea inlet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for estuary that I've seen before include "Type of English" , "A surety found in river's tidal mouth" , "A surety for river mouth" , "River near sea" , "Tidal mouth of a large river" .)