Rugged form of bitterness (6)

I believe the answer is:

grudge

I'm a little stuck... Click here to teach me more about this clue!

'form of bitterness' is the definition.
(I know that grudge is a type of bitterness)

'rugged' is the wordplay.
'rugged' can be anagrammed to 'GRUDGE'.
But, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for grudge that I've seen before include "Resentment, rancour" , "Persistent feeling of resentment" , "Long-standing feeling of resentment" , "Active resentment" , "Feeling of resentment or illwill" .)

I've seen this clue in The Daily Express.

Want a hint initially instead of a full solution? Install my app