Cord restricting knight's weapon (5)
I believe the answer is:
lance
'weapon' is the definition.
(I know that lance is a type of weapon)
'cord restricting knight's' is the wordplay.
'cord' becomes 'lace' (lace is a kind of cord).
'restricting' indicates putting letters inside.
'knight' becomes 'N' (chess abbreviation).
'lace' enclosing 'n' is 'LANCE'.
(Other definitions for lance that I've seen before include "Old cavalry weapon" , "Clean (anag) -- weapon" , "Surgical knife" , "Rider's long weapon" , "Boy - prick (a boil)" .)