Much less isolated with other people visiting (3,5)
I believe the answer is:
let alone
'much less' is the definition.
(I've seen this before)
'isolated with other people visiting' is the wordplay.
'isolated' becomes 'lone' (both can mean alone).
'with other people' becomes 'et al' (short for 'et alia', 'and others' in Latin).
'visiting' is an insertion indicator.
'lone' enclosing 'etal' is 'LET-ALONE'.
(Other definitions for let alone that I've seen before include "Not to mention - much less - avoid" , "Not to mention (thing)" , "Don't bother someone - allow to be solitary" , "Not to mention, never mind" , "Don't touch" .)