Thrust, impetus (7)
I believe the answer is:
impulse
I believe this is a double definition.
'thrust' is the first definition.
(impulse is a kind of thrust)
'impetus' is the second definition.
(I know that impetus can be written as impulse)
(Other definitions for impulse that I've seen before include "Chew" , "Miles up (anag)" , "Whim; thrust" , "A sudden inclination to act (7)" , "Instinctive motive" .)