The sprinter's capacity for a drug (5)

I believe the answer is:

speed

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'for a drug' is the definition.
(speed is a kind of drug)

I cannot understand how the remainder of the clue works.

(Other definitions for speed that I've seen before include "Alacrity" , "'More haste, less . . . . .'" , "Swiftness" , "Rapidity - amphetamine" , "Tempo - drug" .)

I've seen this clue in The Globe and Mail.

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