Goad, incentive (4)
I believe the answer is:
spur
This clue is a double definition.
'goad' is the first definition.
(I know that spur is a type of goad)
'incentive' is the second definition.
(spur can mean an incentive or stimulus)
(Other definitions for spur that I've seen before include "outcrop" , "Cowboy's goading instrument" , "Cowboy boot attachment" , "Spike on a horse-rider's boot" , "offer encouragement" .)