Friend of Opal's (3)
I believe the answer is:
pal
'friend' is the definition.
(pal can mean a friend)
'of opal's' is the wordplay.
'of' indicates the answer is hidden within the clue.
'PAL' is hidden within 'opals'.
(Other definitions for pal that I've seen before include "Mate, informal friend" , "Buddy, chum" , "China" , "Mate, buddy" , "Friend (colloq.)" .)