Irritate, vex (3)
I believe the answer is:
irk
I believe this is a double definition.
'irritate' is the first definition.
(to irk is to irritate)
'vex' is the second definition.
(I've seen this in another clue)
(Other definitions for irk that I've seen before include "Severely annoy" , "upset" , "English river" , "Bug" , "Annoy, bother" .)