Performed as a stand-in (5)
I believe the answer is:
acted
'performed' is the definition.
(acting is a kind of performing)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for acted that I've seen before include "Worked" , "Cadet (anag)" , "Play a role" , "Posed as" , "Feigned" .)