Not to mention going out late and unaccompanied (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')
'going out late and unaccompanied' is the wordplay.
'going out' is an anagram indicator (out can mean wrong or inaccurate).
'and' says to put letters next to each other.
'unaccompanied' becomes 'lone' (I've seen this before).
'late' is an anagram of 'leta'.
'leta'+'lone'='LET-ALONE'
(Other definitions for let alone that I've seen before include "Don't touch" , "Don't bother someone - allow to be solitary" , "Not to mention (thing)" , "Not to mention - much less - avoid" , "Not to mention, never mind" .)