'Slow down!' making you more likely to run? (3,2,4)

I believe the answer is:

not so fast

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'slow down' is the definition.
(as imperatives they mean the same thing)

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

(Other definitions for not so fast that I've seen before include "Wait" , "Stop right there! - Slower!" .)

I've seen this clue in The Everyman puzzle in The Observer.

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