One hunts criminal by river in Cornwall (6)
I believe the answer is:
falcon
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'hunts criminal by river in cornwall' is the wordplay.
I cannot quite understand how this works, but
'criminal' could be 'con' (abbreviation for 'convict') and 'con' is found within the answer.
'river' could be 'fal' (Cornish river) and 'fal' is located in the answer.
No letters remain.
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" , "Hunting bird like peregrine or merlin" , "Fast bird of prey" , "Hunting bird, may be trained" , "that's seen flying" .)