Old friend gets gem (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'old friend' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'gets' is the link.
(Other definitions for opal that I've seen before include "One's stony" , "little jewel" , "Delicately-coloured gem" , "often semi-transparent" , "Rock" .)