A hundred up unlikely? 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')
'a hundred up unlikely?' is the wordplay.
'hundred' becomes 'ton' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'unlikely?' becomes 'tall' (as in a tall tale).
'a'+'ton'='aton'
'aton' back-to-front is 'nota'.
'nota'+'tall'='NOT AT ALL'
(Other definitions for not at all that I've seen before include "Don't mention it!" , "My pleasure" , "forget it" , "You're welcome" , "Never!" .)