Keep changing mind as poorly during leave (9)
I believe the answer is:
vacillate
'keep changing mind' is the definition.
'poorly during leave' is the wordplay.
'poorly' becomes 'ill' ('ill' can be a synonym of 'poorly').
'during' means one lot of letters goes inside another.
'leave' becomes 'vacate' (vacating is a kind of leaving**).
'ill' put into 'vacate' is 'VACILLATE'.
'as' is the link.
(Other definitions for vacillate that I've seen before include "Be in two minds" , "indecisive" , "Hesitate" , "Be irresolute" , "Dither" .)