So, after all, is left unfinished, too (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'so after all is left unfinished' is the wordplay.
'after' says to put letters next to each other.
'is left unfinished' means to remove the last letter.
'all' with its final letter taken off is 'al'.
'so' after 'al' is 'ALSO'.
(Other definitions for also that I've seen before include "revolutionary a part for Yugoslavia" , "item's rarely used now for this" , "Likewise" , "Item" , "on top" .)