Prompt — impel (8)
I believe the answer is:
motivate
This clue is a double definition.
'prompt' is the first definition.
(I know that prompt can be written as motivate)
'impel' is the second definition.
(I've seen this in another clue)
(Other definitions for motivate that I've seen before include "Stimulate to action" , "Give an incentive for action" , "Induce" , "Prompt" , "Stimulate the interest of" .)