So after all left unfinished, too (4)
I believe the answer is:
also
'too' is the definition.
('also' can be a synonym of 'too')
'so after all left unfinished' is the wordplay.
'after' means one lot of letters go next to another.
'left unfinished' means to remove the last letter.
'all' with its last letter taken off is 'al'.
'so' after 'al' is 'ALSO'.
(Other definitions for also that I've seen before include "Old" , "Plus" , "Likewise" , "'Besides, moreover (4)'" , "What's more" .)