Responding about codebreaker concealing note (9)
I believe the answer is:
returning
'responding' is the definition.
(returning is a kind of responding)
'about codebreaker concealing note' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'codebreaker' becomes 'Turing' (WWe codebreaker Alan Turing).
'concealing' is an insertion indicator.
'note' becomes 'n' (abbreviation).
'turing' going around 'n' is 'turning'.
're'+'turning'='RETURNING'
(Other definitions for returning that I've seen before include "Going or taking back" , "Coming back" , "Yielding" , "On the way back" .)