Shallow lake (6)
I believe the answer is:
lagoon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">shallow lake</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lagoon that I've seen before include "Inland sea" , "Shallow seaside lake" , "Pool" , "Area inside atoll" , "Salt-water lake, often blue" .)