The result of adding to an endless story! (5)
I believe the answer is:
total
'the result of adding' is the definition.
(I've seen this in another clue)
'to an endless story' is the wordplay.
'an endless' means to remove the last letter.
'story' becomes 'tale' (synonyms).
'tale' with its final letter taken off is 'tal'.
'to'+'tal'='TOTAL'
(Other definitions for total that I've seen before include "Final tally" , "Complete, absolute" , "Complete, entire amount" , "outright" , "Come to" .)