Undercover agent in car of your dreams? (7)

I believe the answer is:

sleeper

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'undercover' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for sleeper that I've seen before include "One who slumbers" , "Wagon-lit" , "Rail track supporter" , "film not successful at first" , "Form of transport" .)

I've seen this clue in The Independent.

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