Underwater explorer (5)
I believe the answer is:
diver
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater explorer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(someone diving under the sea)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for diver that I've seen before include "Drive (anag.)" , "One jumping off board" , "Underwater worker" , "One plunges head first into water" , "Bird that plunges" .)