Beam at fish (3)
I believe the answer is:
ray
I believe this clue is a double definition.
'beam' is the first definition.
('ray' can be a synonym of 'beam')
'fish' is the second definition.
(ray is a kind of fish)
'at' is the link.
(Other definitions for ray that I've seen before include "A bright beam or flat fish" , "Flat fish or sun beam" , "may be electric" , "Sunbeam or flat fish" , "Man, maybe" .)