It's plain it was the place for ancient games (7)

I believe the answer is:

olympia

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'it's plain' is the definition.
(Olympia is an example)

I can't explain the remainder of the clue.

(Other definitions for olympia that I've seen before include "Greek scene of historic games" , "historic venue for games" , "Dublin theatre" , "Original setting for many events" , "Grecian plain, Zeus's chief sanctuary" .)

I've seen this clue in The Globe and Mail.

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