Standing by decision after performing (2,4)
I believe the answer is:
on call
'standing by' is the definition.
(someone on call is standing by to help)
'decision after performing' is the wordplay.
'decision' becomes 'call' (call is a kind of decision).
'after' means one lot of letters go next to another.
'performing' becomes 'on' (as in being on stage).
'call' after 'on' is 'ON CALL'.
(Other definitions for on call that I've seen before include "Able to be contacted, like a GP" , "Contactable" , "Like a doctor, perhaps" , "ready to respond" , "Available if needed" .)