Hawk about to catch river fish (5)
I believe the answer is:
trout
'river fish' is the definition.
(I've seen this before)
'hawk about to catch' is the wordplay.
'hawk' becomes 'tout' (both can mean to sell something).
'about' is an insertion indicator.
'to catch' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'tout' placed around 'r' is 'TROUT'.
Can you help me to learn more?
(Other definitions for trout that I've seen before include "bad-tempered woman" , "Unattractive old woman" , "Mistiness" , "Schubert work; fish" , "air" .)