Marine ray will eat river fish (3,5)
I believe the answer is:
sea bream
'fish' is the definition.
(sea bream is a kind of fish)
'marine ray will eat river' is the wordplay.
'marine' becomes 'sea' (I've seen this before).
'ray' becomes 'beam' ('beam' can be a synonym of 'ray').
'will eat' means one lot of letters goes inside another.
'river' becomes 'r'.
'beam' enclosing 'r' is 'bream'.
'sea'+'bream'='SEA-BREAM'
(Other definitions for sea bream that I've seen before include "Fish, valued as food" , "Fish - seem Arab (anag)" , "Edible marine fish" .)