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'm unsure of the 'immediate' bit.
'about to act by' is the wordplay.
'about to' becomes 're' (I've seen this in other clues).
'act' becomes 'turn' (theatre).
'by' means one lot of letters go next to another.
'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" .)