Old friend has a jewel (4)
I believe the answer is:
opal
'a jewel' is the definition.
(I've seen this before)
'old friend' is the wordplay.
'old' becomes 'o' (abbreviation).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'has' acts as a link.
(Other definitions for opal that I've seen before include "item in Hatton Garden?" , "raw material" , "little jewel" , "Delicately coloured gem" , "Gem stone" .)