For Greek student, introduction to Thucydides? (5)

I believe the answer is:

theta

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'for greek' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps there's an association between them I don't understand?

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

Can you help me to learn more?

(Other definitions for theta that I've seen before include "Letter from Greece" , "8th letter of the Greek alphabet" , "Homeric character" , "letter to Greeks" , "classic character?" .)

I've seen this clue in The Times.

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