Omani capital — white grape (6)
I believe the answer is:
muscat
This clue is a double definition.
'omani capital' is the first definition.
(capital of Oman)
'white grape' is the second definition.
(I've seen this in another clue)
(Other definitions for muscat that I've seen before include "Variety of grape - capital of Oman" , "Sweet fortified wine" , "Sweet white wine - Omani port" , "Arabic port" , "Source of alcoholic drink" .)