Shark's appendage (3)
I believe the answer is:
fin
'shark's 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 "Fishy appendage" , "Part of an aircraft's tail" , "Swimmer" , "Fish propeller; end (Fr.)" , "Aerodynamic surface" .)