The result of adding to unfinished story (5)
I believe the answer is:
total
'the result of adding' is the definition.
(I've seen this in another clue)
'to unfinished story' is the wordplay.
'unfinished' means to remove the last letter.
'story' becomes 'tale' (synonyms).
'tale' with its final letter taken away is 'tal'.
'to'+'tal'='TOTAL'
(Other definitions for total that I've seen before include "Complete sum" , "Entire sum" , "Damage beyond repair" , "final answer" , "energy provider" .)