Beam of light on a fish? (3)
I believe the answer is:
ray
'beam of light on a fish?' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ray that I've seen before include "Pencil of light" , "Man who engaged in surrealist photography" , "Sunbeam or flat fish" , "Lindwall" , "Shaft of light" .)