Drop long story with no conclusion (3)
I believe the answer is:
sag
'drop' is the definition.
(sagging is a kind of dropping)
'long story with no conclusion' is the wordplay.
'long story' becomes 'saga' (a saga is a long and complicated story).
'with no conclusion' means to remove the last letter.
'saga' with its last letter taken away is 'SAG'.
(Other definitions for sag that I've seen before include "Give in the middle" , "Fail to retain interest" , "Hang down loosely" , "Sinking" , "Droop, flag" .)