Crossword Genius

Spooner’s army marched fast (9)

Ross

I believe the answer is:

posthaste

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'fast' is the definition.
(thesaurus)

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(Other definitions for posthaste that I've seen before include "Quick" , "As quickly as possible" , "With great speed and urgency" , "With great speed, after hurry" , "With particular speed" .)

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