There's no accounting for his performance (7)
I believe the answer is:
amateur
'there's no accounting for his performance' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is the entire clue.
Can you help me to learn more?
(Other definitions for amateur that I've seen before include "A non-professional sportsman" , "One is not professional" , "Inept, non-professional" , "Athlete who performs without payment" , "Non professional, eight across" .)