Harboring a grudge (4)
I believe the answer is:
sore
'harboring a grudge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sore that I've seen before include "Painful spot" , "buck" , "Painful, or the source of pain" , "Wound; painful" , "source of distress" .)