Rex -- Joy's brother perhaps (8)
I believe the answer is:
relation
'brother perhaps' is the definition.
(brother is a kind of relation)
'rex joy's' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'joy' becomes 'elation' (elation is a kind of joy).
'r'+'elation'='RELATION'
(Other definitions for relation that I've seen before include "One of the family" , "Oriental kind of family connection" , "Respect" , "Kinsman; narrative" , "Story; member of family" .)