"Are you sure?" (6)

I think it could be either:

Which length description are you looking for?:

2,4,2

really

'are you sure?' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for really that I've seen before include "Genuinely" , "Actually, truly" , "as a matter of fact" , "Actually, no kidding" , "Honestly, it's a fact" .)

I've seen this clue in the Newsday.

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