Weapons from Mars? (4)
I believe the answer is:
arms
'weapons' is the definition.
('arm' can be a synonym of 'weapon')
'from mars?' is the wordplay.
'from' indicates anagramming the letters (from the letters).
'mars' with letters rearranged gives 'ARMS'.
(Other definitions for arms that I've seen before include "pacifists won't take up" , "Heraldic devices" , "Protesting violently, up in these" , "Members" , "Weapons and limbs" .)