Undoing curse, monarch's saviour (7)
I believe the answer is:
rescuer
'saviour' is the definition.
('rescuer' can be a synonym of 'saviour')
'undoing curse monarch's' is the wordplay.
'undoing' is an anagram indicator.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina**).
'curse' anagrammed gives 'rescu'.
'rescu'+'er'='RESCUER'
(Other definitions for rescuer that I've seen before include "One who saves from a dangerous situation" , "Saviour" , "One who saves someone" , "freer" , "one retrieves" .)