Blackout sees regulars leaving for capital (4)
I believe the answer is:
'for capital' is the definition.
(capital of Azerbaijan)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for baku that I've seen before include "Azerbaijan's capital" , "Capital city of Azerbaijan" , "Caspian Sea port" , "Capital of Azerbaijan on shore of Caspian Sea" .)