Impetus; boost (8)

I believe the answer is:

stimulus

This clue is a double definition.

'impetus' is the first definition.
(thesaurus)

'boost' is the second definition.
(thesaurus)

(Other definitions for stimulus that I've seen before include "Spur or catalyst for action" , "It evokes a reaction" , "Prod or incentive" , "Something provoking a response" , "Trigger" .)

I've seen this clue in the Sydney Morning Herald.

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