Too slow to reveal capital? (4)
I believe the answer is:
oslo
'capital?' is the definition.
(capital of Norway)
'too slow to reveal' is the wordplay.
'to reveal' indicates a hidden word.
'OSLO' can be found hidden inside 'too slow'.
(Other definitions for oslo that I've seen before include "Capital of Norway" , "where Ibsen lived under another name" , "Norwegian chief port" , "Norwegian capital" , "Northern capital of Czechoslovakia?" .)