Labour lefties having impetus (8)

I believe the answer is:

momentum

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'impetus' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for momentum that I've seen before include "Drive" , "Corbyn supporters" , "Impulse" , "Impetus gained by movement, maybe downhill" , "Driving force, impetus" .)

I've seen this clue in The Sun.

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