Capital found on the Thames? (7)

I believe the answer is:

belfast

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'capital' is the definition.
(I know that Belfast is a national capital)

I don't understand the rest of the clue.

(Other definitions for belfast that I've seen before include "City on the Lagan" , "Capital of Northern Ireland" , "N. Ireland city" , "Northern Ireland home to Queen's University" , "NI city" .)

I've seen this clue in The Guardian.

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