At the time, the capital of Norway (4)
I believe the answer is:
then
'at the time' is the definition.
(synonyms)
'the capital of norway' is the wordplay.
'capital of' says to take the initial letters.
The initial letter of 'norway' is 'n'.
'the'+'n'='THEN'
(Other definitions for then that I've seen before include "Once upon a time" , "At that time; next" , "Afterwards - as a result" , "Subsequently - previously" , "soon?" .)