"Are you ___ serious?!" (5)
'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" .)
(Other definitions for being that I've seen before include "See 9" , "Existing" , "A matter of 9 in the play" , "Living thing" , "Human, perhaps" .)