Weapon knight's hidden in fine material (5)
I believe the answer is:
lance
'weapon' is the definition.
(I know that lance is a type of weapon)
'knight's hidden in fine material' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'hidden in' is an insertion indicator.
'fine material' becomes 'lace'.
'n' put within 'lace' is 'LANCE'.
(Other definitions for lance that I've seen before include "Cut open" , "A horse soldier's spear" , "steel" , "Jousting weapon" , "Boy - prick (a boil)" .)