Cartilaginous fish (3)
I believe the answer is:
ray
'cartilaginous fish' is the definition.
(ray is a kind of cartilaginous fish)
This is the entire clue.
(Other definitions for ray that I've seen before include "Beam of light or flat fish" , "Narrow beam of light" , "Man, maybe" , "Beam - skate" , "Second note for the flat fish" .)