Members of Black Sabbath joined famous group (4)
I believe the answer is:
abba
'famous group' is the definition.
(famous pop group)
'members of black sabbath' is the wordplay.
'members of' indicates a hidden word (the letters are 'members' of the clue).
'ABBA' can be found hidden inside 'black sabbath'.
'joined' acts as a link.
This may not be right. It may be part of another bit of the clue.
(Other definitions for abba that I've seen before include "famous pop group" , "Scandinavian supergroup" , "Father (used of God in New Testament) - Swedish band" , "6 24 113 [SCAN-DIN-AVIAN] band" , "Swedish artists" .)