Potentially one’s best friend (5)
I believe the answer is:
china
'friend' is the definition.
(cockney rhyming slang, mate is china plate)
'potentially one's best' is the wordplay.
'potentially one' becomes 'chin' (I am not sure about this - if you are sure you should believe this answer much more).
'best' becomes 'a'.
'chin'+'a'='CHINA'
Can you help me to learn more?
(Other definitions for china that I've seen before include "friend or companion" , "Large country; mate" , "Ceramic; country" , "High quality porcelain" , "cup and saucer?" .)