Become submerged (4)
I believe the answer is:
sink
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become submerged</span>' is the definition.<br/ ><span class="explanation-format__explanation">(sink underwater)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sink that I've seen before include "Washing-up place" , "Descend into water" , "Washbasin" , "Pocket (billiards)" , "Disappear below the surface" .)