Old friend’s a gem (4)
I believe the answer is:
'a gem' is the definition.
(I know that opal is a type of opaque gem)
'old friends' is the wordplay.
'old' becomes 'o' (abbreviation).
'friends' becomes 'pal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Delicately-coloured gem" , "Milky-white gem" , "Rock" , "Quartz-like gemstone" , "item in Hatton Garden?" .)