Labour's left having impetus (8)
I believe the answer is:
momentum
'impetus' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for momentum that I've seen before include "Impelling force" , "Corbynites" , "Corbyn supporters" , "Impetus from motion" , "Impulse" .)