Tidal mouth of a river (7)
I believe the answer is:
estuary
'tidal mouth of a river' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for estuary that I've seen before include "Wide river mouth" , "Tidal mouth of a large river" , "should be apparent at Dartmouth?" , "Bristol Channel?" , "Type of English" .)