Of water (7)
I believe the answer is:
aquatic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of water</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 aquatic that I've seen before include "Growing in or near water" , "Linked to water" , "Pertaining to water" , "Relating to water" , "Marine plant" .)