... not to mention abandoned with no following (3,5)
I believe the answer is:
let alone
'not to mention' is the definition.
(both can be used to refer to another point)
'abandoned with no following' is the wordplay.
'abandoned' becomes 'lone' (I've seen this in another clue).
'with' means one lot of letters go next to another.
'no following' becomes 'leta' (I can't explain this - if you can you should believe this answer much more).
'lone' put after 'leta' is 'LET-ALONE'.
Can you help me to learn more?
(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, never mind" , "Not to mention - much less - avoid" , "Not to mention (thing)" .)