Fish appendage (3)
I believe the answer is:
fin
'fish appendage' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fin that I've seen before include "Swimmer's flipper" , "A fish uses it to steer itself" , "Found on a fish" , "Aerodynamic surface" , "Thin projection for fish to steer itself" .)