Archimedes' shout (6)
I believe the answer is:
eureka
'archimedes' shout' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eureka that I've seen before include "Cry announcing a brilliant discovery" , "Cry of triumph (at a discovery)" , "Excited cry after a discovery" , "Cry of success" , "Archimedes' cry of triumph as he ran naked through the streets of Syracuse?" .)