"Aha!"

I think it could be either:

Which length description are you looking for?:

eureka

'aha' is the definition.
(both can be said when discovering something)

This is all the clue.

(Other definitions for eureka that I've seen before include "Greek exclamation of discovery (6" , "Cry of triumph (at a discovery)" , "Archimedes' shout of triumph - re a uke (anag)" , "I've got it!" , "Archimedes' cry of triumph as he ran naked through the streets of Syracuse?" .)

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

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