Beams at fish (4)
I believe the answer is:
rays
This clue is a double definition.
'beams' is the first definition.
(rays of light)
'fish' is the second definition.
(I've seen this before)
'at' acts as a link.
(Other definitions for rays that I've seen before include "Pencils of light" , "shafts" , "Fish with wing-shaped fins and long tails" , "They swim" , "Flat fish" .)