Tidal river mouth (7)
I believe the answer is:
estuary
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tidal river mouth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)