Don't mention it never (3,2,3)
I believe the answer is:
not at all
I believe this clue is a double definition.
'don't mention it' is the first definition.
(both can mean 'you're welcome')
'never' is the second definition.
(thesaurus)
(Other definitions for not at all that I've seen before include "Don't mention it!" , "In no way" , "denial" , "Never!" , "forget it" .)