Baffled accountant will employ this but it's probably not recoverable (4,5)
I believe the answer is:
lost cause
'this but it's probably not recoverable' is the definition.
The definition suggests a singular noun which matches the answer.
'baffled accountant will employ' is the wordplay.
I cannot really understand how this works, but
'baffled' could be 'lost' (I've seen this before) and 'lost' is found in the answer.
'accountant' could be 'ca' (abbreviation for chartered accountant) and 'ca' is present in the answer.
'employ' could be 'use' (synonyms) and 'use' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lost cause that I've seen before include "Hopeless situation" , "write-off" , "Hopeless endeavour" , "Hopeless case" , "Hopeless campaign" .)