Endlessly long story is flop (3)
I believe the answer is:
sag
'is flop' is the definition.
(flop can mean to sag or droop)
'endlessly long story' is the wordplay.
'endlessly' means to remove the last letter.
'long story' becomes 'saga' (a saga is a long and complicated story).
'saga' with its final letter taken away is 'SAG'.
(Other definitions for sag that I've seen before include "Droop, sink in the middle" , "Decline" , "Give in the middle" , "Lose energy" , "Sink, hang down" .)