Be unsteady, being sick during leave (9)
I believe the answer is:
vacillate
'be unsteady' is the definition.
(I've seen this before)
'sick during leave' is the wordplay.
'sick' becomes 'ill' (synonyms).
'during' is an insertion indicator.
'leave' becomes 'vacate' (to vacate a place is to leave it).
'ill' going into 'vacate' is 'VACILLATE'.
'being' is the link.
(Other definitions for vacillate that I've seen before include "Hesitate" , "Keep changing one's mind" , "Shilly-shally" , "Be in two minds" , "Be irresolute" .)