"Are you serious?" (4)

I think it could be either:

Which length description are you looking for?:

what

'are you serious?' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for what that I've seen before include "Word used in a question" , "I beg your pardon" , "I didn't hear" , "That which - come again!" , "Interrogatory word" .)

I've seen this clue in the LA Times and the Newsday.

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