Save energy with curse undone (6)
I believe the answer is:
rescue
'save' is the definition.
(rescuing is a kind of saving)
'energy with curse undone' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'with' is a charade indicator (letters next to each other).
'undone' is an anagram indicator.
'curse' anagrammed gives 'rescu'.
'e' after 'rescu' is 'RESCUE'.
(Other definitions for rescue that I've seen before include "Set free from harm and evil" , "Free from danger or captivity" , "Retrieve" , "Pull from danger" , "Save someone from a dangerous situation" .)