Like Jane's flat? (5)

I believe the answer is:

plain

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'flat?' is the definition.
(flat is a kind of plain)

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

(Other definitions for plain that I've seen before include "Uncomplicated" , "Sheer; frank" , "Simple or apparent" , "Unadorned; evident" , "not striking" .)

I've seen this clue in the Evening Standard.

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