Twice reduced by 50% (4)
I believe the answer is:
once
'twice' is the definition.
Both the definition and answer are adverbs. Perhaps there's a link between them I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for once that I've seen before include "Cone (anag.)" , "Just as a single occurrence" , "In times past" , "In former times" , "in bygone era" .)