Untraditional playing for a start - that's plain (6)

I believe the answer is:

tundra

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'that's plain' is the definition.
(tundra is a kind of plain)

I cannot understand how the remainder of the clue works.

(Other definitions for tundra that I've seen before include "Cold northern region" , "where you might see caribou" , "Cold plain" , "Treeless frozen Arctic plain" , "Arctic region of permafrost" .)

I've seen this clue in the Evening Standard.

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