Ray swallowing river fish (5)
I believe the answer is:
bream
'fish' is the definition.
(I know that bream is a type of fish)
'ray swallowing river' is the wordplay.
'ray' becomes 'beam' (synonyms).
'swallowing' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'beam' enclosing 'r' is 'BREAM'.
(Other definitions for bream that I've seen before include "Food fish" , "Amber (anag) - fish" , "Fish found in amber" , "Edible fish" , "Amber kind of fish" .)