Golf instructor not playing quickly (6)
I believe the answer is:
pronto
'quickly' is the definition.
(I've seen this before)
'golf instructor not playing' is the wordplay.
'golf instructor' becomes 'pro' (I've seen this in another clue).
'playing' is an anagram indicator.
'not' anagrammed gives 'nto'.
'pro'+'nto'='PRONTO'
(Other definitions for pronto that I've seen before include "Quickly, amigo" , "Immediately, amigo" , "Right now, amigo" , "Quickly (sl.)" , "Quick as you can, amigo" .)