I have found it in a Greek dictionary (6)

I believe the answer is:

eureka

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'i' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for eureka that I've seen before include "Exclamation on discovering something" , "Bingo!" , "Greek exclamation of discovery (6" , "Triumphant exclamation of discovery" , "Cry of triumph (at a discovery)" .)

I've seen this clue in The Globe and Mail.

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