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' means one lot of letters go next to another.
'is left unfinished' means to remove the last letter.
'all' with its last letter removed 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" .)