Much less (3,5)
I believe the answer is:
let alone
'much less' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for let alone that I've seen before include "Don't bother someone - allow to be solitary" , "Not to mention, never mind" , "Not to mention (thing)" , "Don't touch" , "Not to mention - much less - avoid" .)