Boast over knight's honour (5)
I believe the answer is:
crown
'honour' is the definition.
(crown is a kind of honour)
'boast over knight's' is the wordplay.
'boast' becomes 'crow' (crow is a kind of boast).
'over' says to put letters next to each other.
'knight' becomes 'n' (chess).
'crow'+'n'='CROWN'
(Other definitions for crown that I've seen before include "The sovereign" , "Coin; royal headgear" , "piece of tooth" , "Symbol of monarchy" , "Monarch's headgear" .)