Grenade primarily a French weapon (3)
I believe the answer is:
gun
'weapon' is the definition.
(I know that gun is a type of weapon)
'grenade primarily a french' is the wordplay.
'primarily' says to take the initial letters.
'a french' becomes 'un' ('a' in French).
The initial letter of 'grenade' is 'g'.
'g'+'un'='GUN'
(Other definitions for gun that I've seen before include "What has barrel" , "Hired killer" , "Throttle" , "Firearm" , "An offensive weapon" .)