Weapon breaks armistice endlessly (8)
I believe the answer is:
scimitar
'weapon' is the definition.
(scimitar is a kind of weapon)
'breaks armistice endlessly' is the wordplay.
'breaks' indicates anagramming the letters.
'endlessly' means to remove the last letter.
'armistice' with its final letter taken away is 'armistic'.
'armistic' with letters rearranged gives 'SCIMITAR'.
(Other definitions for scimitar that I've seen before include "Curved form of sword" , "Curved oriental sabre" , "Steel" , "Type of sword with curved blade" , "Eastern sword" .)