Dolphin feature (3)
I believe the answer is:
fin
'dolphin feature' 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 "Fishy appendage" , "Found on a fish" , "Fish propeller; end (Fr.)" , "Piscine appendage" , "A fish uses it to steer itself" .)