Light fish (3)
I believe the answer is:
ray
This clue is a double definition.
'light' is the first definition.
(I know that ray is a type of light)
'fish' is the second definition.
(I know that ray is a type of fish)
(Other definitions for ray that I've seen before include "Shaft of light" , "Flash < Movie Player" , "may be electric" , "Beam; flat-bodied fish" , "Beam of light or flat fish" .)