Not a great performer, but he has been capped! (5)
I believe the answer is:
dunce
'not a great performer but he has been capped' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he has been capped' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'dunc' and 'dunc' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for dunce that I've seen before include "Dullard" , "Poor student" , "Dolt" , "One slow to pick" , "Old word for person slow at learning" .)