Weaken, to cut a long story short (3)
I believe the answer is:
sag
'weaken' is the definition.
(I've seen this in another clue)
'cut a long story short' is the wordplay.
'cut' is an anagram indicator (cut can mean mixed).
'long story' becomes 'saga' (a saga is a long and complicated story).
'short' indicates named letters should be taken away (to be short of something is to lack it).
'saga' with 'a' removed is 'sga'.
'sga' with letters rearranged gives 'SAG'.
'to' is the link.
(Other definitions for sag that I've seen before include "Lose energy" , "Slump" , "Give in the middle" , "Droop, sink in the middle" , "fall" .)