Sardonic nob said to be a hopeless case (5-3)
I believe the answer is:
write-off
'case' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for write-off that I've seen before include "Cancel, a debt perhaps" , "Cancel like bad debt" , "Send a letter - cancel a debt" , "Car too costly to repair" , "Give up on, recognise as lost like bad debt" .)