Some light, flat-bodied fish (3)
I believe the answer is:
ray
This clue is a double definition.
'some light' is the first definition.
(ray is a kind of light)
'flat-bodied fish' is the second definition.
(I've seen this before)
(Other definitions for ray that I've seen before include "Broad flat fish; beam" , "Pencil of light" , "Beam or flatfish" , "Flatfish or sunbeam" , "Shaft of light" .)