Crossword Genius

Stupid fellow, but he may be on the ball (4)

Ross

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" .)

I've seen this clue in the Evening Standard.
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