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' (a routine or performance).
'by' says to put 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" .)