Weapon pierces a breastplate to some extent (5)
I believe the answer is:
sabre
'weapon' is the definition.
(sabre is a kind of weapon)
'pierces a breastplate to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'SABRE' is hidden in the letters of 'pierces a breastplate'.
(Other definitions for sabre that I've seen before include "Walt --, cartoonist" , "Sword with curved blade" , "Type of sword" , "Slasher" , "Bares (anag.)" .)