Are you sober? One may still find you're over the limit (5,3)

I believe the answer is:

speed cop

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'one may still find you're over the limit' is the definition.
(exceeding the speed limit)

I cannot understand how the remainder of the clue works.

(Other definitions for speed cop that I've seen before include "Policeman" , "Police officer on a motorcycle? (slang)" , "Traffic enforcer" .)

I've seen this clue in The Guardian.

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