About to act by immediate response (6)
I believe the answer is:
return
'immediate response' is the definition.
'return' can be an answer for 'response' (return is a kind of response). I am unsure of the 'immediate' bit.
'about to act by' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to act' becomes 'turn' (theatre).
'by' is a charade indicator (letters next to each other).
're'+'turn'='RETURN'
Can you help me to learn more?
(Other definitions for return that I've seen before include "second half of journey" , "election to parliament" , "Interest" , "Yield" , "Come/go back" .)