Reorganisation of NATO unlikely, so don't mention it (3,2,3)
I believe the answer is:
not at all
'don't mention it' is the definition.
(both can mean 'you're welcome')
'reorganisation of nato unlikely' is the wordplay.
'reorganisation of' indicates an anagram (I've seen this in other clues).
'unlikely' becomes 'tall' (as in a tall tale).
'nato' is an anagram of 'nota'.
'nota'+'tall'='NOT AT ALL'
'so' is the link.
(Other definitions for not at all that I've seen before include "denial" , "acceptance of apology" , "You're welcome" , "Anything but!" , "My pleasure" .)