Hawk brazenly catching river fish (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a type of fish)
'hawk brazenly catching river' is the wordplay.
'hawk brazenly' becomes 'tout' (I've seen this before).
'catching' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'tout' enclosing 'r' is 'TROUT'.
(Other definitions for trout that I've seen before include "Swimmer" , "Unattractive old woman" , "Food fish" , "Tickled fish?" , "Rainbow, say" .)