Tidal river mouth (7)
I believe the answer is:
estuary
'tidal river mouth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for estuary that I've seen before include "Where tide meets stream" , "Tidal mouth of a large river" , "Wide river-mouth" , "A surety for river mouth" , "eg Thames, Humber" .)