Elf’s grudge, harbouring resistance (6)
I believe the answer is:
sprite
'elf's' is the definition.
(I've seen this before)
'grudge harbouring resistance' is the wordplay.
'grudge' becomes 'spite' (I've seen this in another clue).
'harbouring' indicates putting letters inside.
'resistance' becomes 'r' (electronics abbreviation).
'spite' enclosing 'r' is 'SPRITE'.
(Other definitions for sprite that I've seen before include "A fairy or goblin" , "Supernatural being" , "Elf, fairy" , "sports car" , "Brownie" .)