A British citizen is the theme (7)

I believe the answer is:

subject

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This clue could be a double definition.

'a british citizen' is the first definition.
'subject' can be an answer for 'citizen' (I've seen this before). I am not certain of the 'a british' bit.

'the theme' is the second definition.
(synonyms)

'is' is the link.

Can you help me to learn more?

(Other definitions for subject that I've seen before include "Theme (of a fugue)" , "Theme, topic" , "Person under 26?" , "School topic" , "Branch of knowledge" .)

I've seen this clue in The Guardian.

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