Political group’s instant sign of hesitation (8)

I believe the answer is:

momentum

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'political group's instant sign of hesitation' is the definition.
I can't judge whether this defines the answer.

'instant sign of hesitation' is the wordplay.
'instant' becomes 'moment' (synonyms).
'sign of hesitation' becomes 'um' (expression of hesitation).
'moment'+'um'='MOMENTUM'

Can you help me to learn more?

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

I've seen this clue in The Times and The Sun.

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