Heat spots develop quickly (9)
I believe the answer is:
posthaste
'quickly' is the definition.
(thesaurus)
'heat spots develop' is the wordplay.
'develop' indicates an anagram.
'heat'+'spots'='heatspots'
'heatspots' with letters rearranged gives 'POSTHASTE'.
(Other definitions for posthaste that I've seen before include "Yes!" , "With utmost speed -- to the pass (anag)" , "With great speed, after hurry" , "Hurry the mail with utmost speed" , "As quickly as possible" .)