Popular incentive for knight (7)
I believe the answer is:
hotspur
'for knight' is the definition.
(nickname of Henry Percy, 14th-century knight)
'popular incentive' is the wordplay.
'popular' becomes 'hot' (similar in meaning).
'incentive' becomes 'spur' (I've seen this in another clue).
'hot'+'spur'='HOTSPUR'
(Other definitions for hotspur that I've seen before include "Daredevil" , "Fiery type" , "Impetuous, fiery person - nickname for Henry Percy" , "Violent and rash chap" , "named in London team?" .)