Crossword Genius

EastEnders tough guy is upset — that's plain (6)

Ross

I believe the answer is:

tundra

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

I don't understand the remainder of the clue.

(Other definitions for tundra that I've seen before include "Vast flat treeless zone" , "Vast treeless plane" , "Vast level Arctic region with frozen subsoil" , "Area of permafrost" , "Northern permafrost region" .)

I've seen this clue in The Guardian.
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