Vacillate, hesitate (6)
I believe the answer is:
dither
I believe this clue is a double definition.
'vacillate' is the first definition.
(I've seen this in another clue)
'hesitate' is the second definition.
(I've seen this before)
(Other definitions for dither that I've seen before include "Vacillate, be indecisive" , "Waver, vacillate" , "Hesitate indecisively, vacillate" , "Vacillate, be undecided" , "quake" .)