Knight in virtue gives agreement (6)
I believe the answer is:
assent
'agreement' is the definition.
(assent is a kind of agreement)
'knight in virtue' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'in' is an insertion indicator.
'virtue' becomes 'asset' (I've seen this before).
'n' put inside 'asset' is 'ASSENT'.
'gives' acts as a link.
(Other definitions for assent that I've seen before include "Agree - concede" , "Accede, acquiesce" , "Say yes" , "Accede, agree" , "Agree and concur" .)