Fish part (3)
'fish part' is the definition.
('fin' is part of 'fish')
This is all the clue.
(Other definitions for fin that I've seen before include "Swimmer" , "Part of an aircraft's tail" , "A fish uses it to steer itself" , "Paddle-like organ" , "Fishy appendage" .)
(Other definitions for dorsal fin that I've seen before include "Projection on back of fish" , "bit of fish" , "swimmer? Just a bit" , "All that may be seen of shark" , "Stabilising appendage on a fish's back" .)