Something worn for warmth by an ancient Iranian (7)

I believe the answer is:

avestan

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'iranian' is the definition.
(I know that Avestan is a type of iranian)

I don't understand the remainder of the clue.

(Other definitions for avestan that I've seen before include "Thus spake Zarathustra" , "connected with sacred writings" , "relating to ancient Iranian language" .)

I've seen this clue in The Times and The Sunday Times.

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