Impetus, spur (8)
I believe the answer is:
stimulus
I believe this clue is a double definition.
'impetus' is the first definition.
(thesaurus)
'spur' is the second definition.
(I've seen this before)
(Other definitions for stimulus that I've seen before include "That which produces a response" , "Fillip" , "Spur or catalyst for action" , "Something provoking a reaction" , "Trigger" .)