Impetus (8)
'impetus' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for momentum that I've seen before include "Corbyn supporters" , "Forward impetus" , "Impulse" , "Impetus gained by movement, maybe downhill" , "Corbynites" .)
'impetus' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for drive that I've seen before include "Impel; house approach" , "Force into action, impel" , "Steer; trip" , "Sales campaign - energy - push" , "long golf shot" .)