In favour of not changing quickly (6)
I believe the answer is:
pronto
'quickly' is the definition.
(I've seen this before)
'in favour of not changing' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'changing' indicates an anagram.
'not' anagrammed gives 'nto'.
'pro'+'nto'='PRONTO'
(Other definitions for pronto that I've seen before include "Double quick!" , "Quick as you can, amigo" , "Proton (anag.)" , "Quickly, promptly" , "Immediately, amigo" .)