... not to mention being terribly late and solitary (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')
'terribly late and solitary' is the wordplay.
'terribly' indicates anagramming the letters.
'and' is a charade indicator (letters next to each other).
'solitary' becomes 'lone' ('lone' can be a synonym of 'solitary').
'late' with letters rearranged gives 'leta'.
'leta'+'lone'='LET-ALONE'
'being' acts as a link.
(Other definitions for let alone that I've seen before include "Not to mention - much less - avoid" , "Don't touch" , "Not to mention, never mind" , "Not to mention (thing)" , "Don't bother someone - allow to be solitary" .)