Swim in the nude (6,3)
I believe the answer is:
skinny dip
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">swim in the nude</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 skinny dip that I've seen before include "crawl nude?" , "Duck without dressing" .)