Will-power (8)
I believe the answer is:
volition
This clue is a double definition.
'will' is the first definition.
(I know that will can be written as volition)
'power' is the second definition.
(volition is a kind of power)
(Other definitions for volition that I've seen before include "Act of exercising the will" , "Power of using one's will" , "The act of makiing a choice" , "Free will" , "(Exercise of) will" .)