Brigade's capital (4)

I believe the answer is:

riga

I'm a little stuck... Click here to teach me more about this clue!

'capital' is the definition.
(capital of Latvia)

'brigade's' is the wordplay.
'RIGA' is hidden within the letters.
But, I'm unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for riga that I've seen before include "Latvian port" , "port in Latvia" , "Baltic capital city" , "Baltic port" , "A rig in Latvia" .)

I've seen this clue in The Mirror.

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