Stupid fellow, but he may be on the ball (4)
I believe the answer is:
goof
This could be a double definition.
'stupid fellow' is the first definition.
'he may be on the ball' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'but' is the link.
(Other definitions for goof that I've seen before include "Blunder, mistake (slang)" , "foolish error" , "(Make a) silly mistake" , "boob" , "Foolish mistake" .)