I have found it! (6)

I believe the answer is:

eureka

(Other definitions for eureka that I've seen before include "Archimedes's call of triumph" , "Archimedes' cry of triumph as he ran naked through the streets of Syracuse?" , "Archimedes's cry of discovery" , "Greek cry of discovery by Archimedes" , ""I've discovered it!"" .)

I've seen this clue in the Evening Standard and The Mirror.

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