Rebel UN nation holds back repeal (5)
I believe the answer is:
annul
'repeal' is the definition.
(synonyms)
'rebel un nation holds back' is the wordplay.
'holds' indicates the answer is hidden within the clue.
'back' says the letters should be written backwards.
'lunna' can be found hidden inside 'rebel un nation'.
'lunna' written backwards gives 'ANNUL'.
(Other definitions for annul that I've seen before include "Invalidate" , "Cancel, revoke" , "Declare (a law) invalid" , "Scrap" , "scratch" .)