Reject incentive offered by knight (5)
I believe the answer is:
spurn
'reject' is the definition.
(I've seen this before)
'incentive offered by knight' is the wordplay.
'incentive' becomes 'spur' (I've seen this before).
'offered by' means one lot of letters go next to another.
'knight' becomes 'n' (chess).
'spur'+'n'='SPURN'
(Other definitions for spurn that I've seen before include "Cold-shoulder" , "Despise" , "Scorn" , "Refuse with contempt" , "Reject scornfully, say unwanted suitor" .)