Result of putting one's foot down? (5)

I believe the answer is:

speed

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'foot down?' is the definition.
'speed' can be an answer for 'foot' (thesaurus). I'm not certain of the 'down?' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for speed that I've seen before include "Move quickly (and illegally?)" , "Dispatch" , "'More haste, less . . . . .'" , "Alacrity" , "make rapid progress" .)

I've seen this clue in the Evening Standard.

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