Disturb; fluster (4)
I believe the answer is:
faze
This clue is a double definition.
'disturb' is the first definition.
(I've seen this before)
'fluster' is the second definition.
(I've seen this in another clue)
(Other definitions for faze that I've seen before include "Be disturbing" , "Disconcert, worry" , "Throw from" , "Disturb, disconcert" , "disconcerting" .)