Plain, but with incremental stages? Not entirely (6)

I believe the answer is:

steppe

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'plain' is the definition.
(steppe is a kind of plain)

I cannot understand how the rest of the clue works.

(Other definitions for steppe that I've seen before include "crosses Siberia" , "plain territory" , "Large treeless area of Siberia" , "Dry grassy plain of central Europe" , "Treeless plain of Russia" .)

I've seen this clue in The Independent.

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