Fin on the back of a fish (6)
I believe the answer is:
dorsal
'fin on the back of a fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dorsal that I've seen before include "Fin on the back of marine vertebrates" , "(Of fin) on upper side" , "'On the back, a fin on a fish perhaps (6)'" , "On the upper back" , "On the back, fin on a fish say" .)