Capital of Czechoslovakia (4)
I believe the answer is:
'capital' is the definition.
(capital of Norway)
'of czechoslovakia' is the wordplay.
'of' says the answer is hidden in the clue.
'OSLO' is hidden within 'czechoslovakia'.
(Other definitions for oslo that I've seen before include "Norwegian capital" , "Capital in throes of go-slow" , "Location" , "Scandinavian port" , "Northern capital of Czechoslovakia?" .)