Resentment is strangely rugged (6)
I believe the answer is:
grudge
'resentment' is the definition.
(I know that grudge is a type of resentment)
'strangely rugged' is the wordplay.
'strangely' indicates anagramming the letters.
'rugged' is an anagram of 'GRUDGE'.
'is' acts as a link.
(Other definitions for grudge that I've seen before include "Persistent feeling of resentment" , "Long-standing feeling of resentment" , "Feeling of resentment or illwill" , "Envy or bad feeling towards" , "Hard feelings" .)