"Aha!"
'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?" .)