Not to mention the tale Noel fabricated (3,5)
I believe the answer is:
let alone
'not to mention' is the definition.
('let alone' can be a synonym of 'not to mention')
'the tale noel fabricated' is the wordplay.
'fabricated' indicates anagramming the letters.
'tale'+'noel'='talenoel'
'talenoel' with letters rearranged gives 'LET-ALONE'.
(Other definitions for let alone that I've seen before include "Not to mention, never mind" , "Don't touch" , "Not to mention - much less - avoid" , "Don't bother someone - allow to be solitary" , "Not to mention (thing)" .)