Irritate, fester (6)
I believe the answer is:
rankle
I believe this clue is a double definition.
'irritate' is the first definition.
(I know that rankle is a more specific form of the action irritate)
'fester' is the second definition.
(I've seen this before)
(Other definitions for rankle that I've seen before include "Continue to vex" , "Go on irritating" , "keep hurting" , "Cause lasting bitterness and resentment" , "Continue to cause annoyance or bitterness" .)