Crossword Genius

Diminished capital, formerly (3)

Ross

I believe the answer is:

rio

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'diminished capital' is the definition.
'rio' can be an answer for 'capital' (I've seen this before). I'm not certain of the 'diminished' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for rio that I've seen before include "old capital" , "Port, -- de Janeiro" , "A Mexican river - like Grande" , "(Golfer) in the sand" , "The ... Grande borders Texas and Mexico" .)

I've seen this clue in the Evening Standard.
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