Weapon provided in low joint, mainly (5)
I believe the answer is:
knife
'weapon' is the definition.
(knife is a kind of weapon)
'provided in low joint mainly' is the wordplay.
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'in' means one lot of letters goes inside another.
'low joint' becomes 'knee' (joint of the body).
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'knee' with its final letter removed is 'kne'.
'if' going inside 'kne' is 'KNIFE'.
(Other definitions for knife that I've seen before include "Sharp tool" , "Table implement" , "Cutting instrument" , "following Stanley perhaps" , "Cutting tool" .)