Gem provided by old friend (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of gem)
'by old friend' is the wordplay.
'by' means one lot of letters go next to another.
'old' becomes 'o' (abbreviation).
'friend' becomes 'pal' (pal can mean a friend).
'o' put next to 'pal' is 'OPAL'.
'provided' is the link.
(Other definitions for opal that I've seen before include "Opaque mineral used a gemstone" , "Milk-coloured gemstone" , "Shimmering stone" , "Semi-precious stone of b;uish white colour" , "Milky-white mineral" .)