Impetus in Labour's left? (8)

I believe the answer is:

momentum

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'impetus' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

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

I've seen this clue in The Sun.

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