Nato reorganisation unlikely, but don't mention it (3,2,3)
I believe the answer is:
not at all
'don't mention it' is the definition.
(I've seen this before)
'nato reorganisation unlikely' is the wordplay.
'reorganisation' indicates anagramming the letters.
'unlikely' becomes 'tall' (I've seen this in other clues).
'nato' with letters rearranged gives 'nota'.
'nota'+'tall'='NOT AT ALL'
'but' acts as a link.
(Other definitions for not at all that I've seen before include "Anything but!" , "acceptance of apology" , "By no means - polite disclaimer" , "forget it" , "You're welcome" .)