Go slowly ... jams in capital city abroad (4)
I believe the answer is:
oslo
'capital city abroad' is the definition.
I can't judge whether this defines the answer.
'go slowly jams in' is the wordplay.
'in' says the answer is hidden in the clue.
'OSLO' is hidden within 'go slowly jams'.
Can you help me to learn more?
(Other definitions for oslo that I've seen before include "where Ibsen lived under another name" , "City in Norway" , "Norwegian capital" , "Northern capital of Czechoslovakia?" , "Capital of Norway" .)