Fish propeller (3)
I believe the answer is:
fin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fish propeller</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for fin that I've seen before include "Aerodynamic surface" , "Found on a fish" , "Swimmer's flipper" , "Swimmer" , "Paddle-like organ" .)