Crossword Genius

Labour's left having impetus (8)

Ross

I believe the answer is:

momentum

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

I don't understand the remainder of the clue.

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(Other definitions for momentum that I've seen before include "Impelling force" , "Corbynites" , "Corbyn supporters" , "Impetus from motion" , "Impulse" .)

I've seen this clue in The Sun.
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