Rugged form of resentment (6)
I believe the answer is:
grudge
'resentment' is the definition.
(grudge is a kind of resentment)
'rugged form' is the wordplay.
'form' indicates an anagram.
'rugged' with letters rearranged gives 'GRUDGE'.
'of' acts as a link.
(Other definitions for grudge that I've seen before include "Resentment, animosity" , "Feeling of enmity or envy" , "Resent having to give or acknowledge something" , "Long-standing feeling of resentment" , "Envy or bad feeling towards" .)