Blow hot and cold as sick during leave (9)
I believe the answer is:
vacillate
'blow hot and cold' is the definition.
(thesaurus)
'sick during leave' is the wordplay.
'sick' becomes 'ill' (similar in meaning).
'during' means one lot of letters goes inside another.
'leave' becomes 'vacate' (to vacate a place is to leave it).
'ill' inserted into 'vacate' is 'VACILLATE'.
'as' acts as a link.
(Other definitions for vacillate that I've seen before include "Hesitate" , "Be irresolute" , "Be unsteady" , "Keep changing one's mind" , "Dither" .)