Spots knight going for a whizz (3)
I believe the answer is:
ace
'for a whizz' is the definition.
('ace' can be a synonym of 'whizz')
'spots knight going' is the wordplay.
'spots' becomes 'acne' (skin condition causing spots).
'knight' becomes 'n' (chess abbreviation).
'going' indicates named letters should be taken away.
'acne' with 'n' removed is 'ACE'.
(Other definitions for ace that I've seen before include "Test pilot'" , "Player who excels" , "One or 11 in cards" , "Tennis serve that the receiver can't reach" , "Crack" .)