Weapon has space to threaten (6)
I believe the answer is:
menace
'to threaten' is the definition.
('menace' can be a synonym of 'threaten')
'weapon has space' is the wordplay.
'weapon' becomes 'mace'.
'has' is an insertion indicator.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'mace' going around 'en' is 'MENACE'.
(Other definitions for menace that I've seen before include "A threatening quality" , "Act in a threatening manner" , "peril" , "Threat; threaten" , "Intimidation" .)