Shark appendage (3)
I believe the answer is:
fin
'shark appendage' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fin that I've seen before include "Swimmer" , "Thin projection for fish to steer itself" , "A fish uses it to steer itself" , "Paddle-like organ" , "Fish propeller; end (Fr.)" .)