Citizen shows topic for discussion (7)

I believe the answer is:

subject

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'citizen' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for subject that I've seen before include "A national - topic of discussion" , "Theme (of a fugue)" , "In school possibly English" , "issue" , "School topic" .)

I've seen this clue in the Sydney Morning Herald.

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