At the bitter end, throes can be briefer (7)
I believe the answer is:
shorter
'briefer' is the definition.
('short' can be a synonym of 'brief')
'at the bitter end throes can be' is the wordplay.
'at' is a charade indicator (letters next to each other).
'end' says to take the final letters.
'can be' indicates anagramming the letters.
The final letter of 'bitter' is 'r'.
'r'+'throes' is 'rthroes'.
'rthroes' with letters rearranged gives 'SHORTER'.
(Other definitions for shorter that I've seen before include "Lower in stature" , "More limited in duration" , "Brusquer" , "More lacking in length" , "Not so tall" .)