Droop as long story appears endless (3)
I believe the answer is:
sag
'droop' is the definition.
(I know that droop can be written as sag)
'long story appears endless' is the wordplay.
'long story' becomes 'saga' (a saga is a long and complicated story).
'appears endless' means to remove the last letter.
'saga' with its last letter removed is 'SAG'.
'as' is the link.
(Other definitions for sag that I've seen before include "Droop, flag" , "Lose energy" , "Cave in" , "Sink (under load)" , "Give way under pressure" .)