Deadly but not hard, one is much less (3,5)

I believe the answer is:

let alone

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'much less' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for let alone that I've seen before include "Not to mention (thing)" , "Not to mention - much less - avoid" , "Don't touch" , "Don't bother someone - allow to be solitary" , "Not to mention, never mind" .)

I've seen this clue in The Hindu.

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