Trick to get beneath river bird (6)
I believe the answer is:
falcon
'bird' is the definition.
(falcon is a kind of bird)
'trick to get beneath river' is the wordplay.
I cannot quite see how this works, but
'trick' could be 'con' (a con is a fraud or trick) and 'con' is found within the answer.
'river' could be 'fal' (River Fal) and 'fal' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for falcon that I've seen before include "Hunting bird, may be peregrine" , "Diving bird of prey" , "that's seen flying" , "See 16" , "high-flyer" .)