Friend of a great power (5)
I believe the answer is:
china
I believe this is a double definition.
'friend' is the first definition.
(cockney rhyming slang, mate is china plate)
'a great power' is the second definition.
'of' acts as a link.
(Other definitions for china that I've seen before include "Oriental country" , "Country - crockery" , "Ceramic ware" , "Household ceramics" , "Ceramic material" .)