Starts to believe as King unites capital (4)
I believe the answer is:
baku
'capital' is the definition.
(capital of Azerbaijan)
'starts to believe as king unites' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'believe as king unites' is 'BAKU'.
(Other definitions for baku that I've seen before include "Caspian Sea port" , "Capital of Azerbaijan on shore of Caspian Sea" , "Azerbaijan's capital" , "Capital city of Azerbaijan" .)