Water plant (4)
I believe the answer is:
reed
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">water plant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(reeds grow in water)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for reed that I've seen before include "Clarinet part; plant" , "Vibrating part of some instruments" , "Rush" , "Instrument; tall grass" , "Riverside grass" .)