Gems for old friends (5)
I believe the answer is:
opals
'gems' is the definition.
(opal is a kind of gem)
'old friends' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'friends' becomes 'pals' (pal is a kind of friend).
'o'+'pals'='OPALS'
'for' acts as a link.
(Other definitions for opals that I've seen before include "Translucent gemstones" , "jewellery" , "Milky sparkling gemstones" , "Semi-precious stones of milky or bluish colour" , "Precious stones" .)