Fish; beam (3)
I believe the answer is:
ray
I believe this clue is a double definition.
'fish' is the first definition.
(I know that ray is a type of fish)
'beam' is the second definition.
(synonyms)
(Other definitions for ray that I've seen before include "Beam of light or flat fish" , "A bright beam or flat fish" , "may be electric" , "Narrow beam of light" , "Shaft of light" .)