Hawk catching river fish (5)
I believe the answer is:
trout
'fish' is the definition.
(trout is a kind of fish)
'hawk catching river' is the wordplay.
'hawk' becomes 'tout' (I've seen this before).
'catching' indicates putting letters inside.
'river' becomes 'r'.
'tout' going around 'r' is 'TROUT'.
(Other definitions for trout that I've seen before include "Swimmer" , "Tasty fish, brown or rainbow" , "Oily fish" , "Fish of salmon family" , "grumpy old woman" .)