Fish in river swallowed by ray (5)
I believe the answer is:
bream
'fish' is the definition.
(bream is a kind of fish)
'river swallowed by ray' is the wordplay.
'river' becomes 'r'.
'swallowed by' means one lot of letters goes inside another.
'ray' becomes 'beam' ('beam' can be a synonym of 'ray').
'r' placed within 'beam' is 'BREAM'.
'in' acts as a link.
(Other definitions for bream that I've seen before include "Fish - British classical guitarist, b. 1933" , "Edible fish" , "Amber (anag) - fish" , "Fish found in amber" , "Swimmer" .)