Impetuous pair lost motivation (7)

I believe the answer is:

impetus

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'motivation' is the definition.
(impetus can mean a motivation or stimulus)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for impetus that I've seen before include "Momentum, stimulus" , "Incentive - time's up (anag)" , "Incentive, stimulus" , "Motivation" , "Driving force, momentum" .)

I've seen this clue in The Daily Express.

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