Collusion: accountant sounds warning (7)
I believe the answer is:
cahoots
'collusion' is the definition.
(to be 'in cahoots' is to collude with someone)
'accountant sounds warning' is the wordplay.
I cannot really see how this works, but
'accountant' could be 'ca' (abbreviation for chartered accountant) and 'ca' is located in the answer.
'sounds' could be 'hoots' (sounds made by owls) and 'hoots' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cahoots that I've seen before include "(In) conspiracy" , "Collusion" , "League" , "In such agreement" , "In - means colluding" .)