Major city in Czechoslovakia (4)

I believe the answer is:

oslo

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'major city' is the definition.
'Oslo' can be an answer for 'city' (Oslo is an example). I am not certain of the 'major' bit.

'in czechoslovakia' is the wordplay.
'in' says the answer is hidden in the clue.
'OSLO' is hidden in the letters of 'czechoslovakia'.

Can you help me to learn more?

(Other definitions for oslo that I've seen before include "Capital in throes of go-slow" , "Nordic city" , "Nordic capital" , "Solo in Norway?" , "where Ibsen lived under another name" .)

I've seen this clue in The Mirror.

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