Fluster; rattle
I believe the answer is:
faze
I believe this is a double definition.
'fluster' is the first definition.
(I've seen this in another clue)
'rattle' is the second definition.
(both can mean to bother or unsettle)
(Other definitions for faze that I've seen before include "Throw from" , "Shake" , "Disturb, disconcert" , "Unsettle - perturb" , "Put off" .)