Old chum is a gem (4)
I believe the answer is:
opal
'a gem' is the definition.
(opal is a kind of gem)
'old chum' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chum' becomes 'pal' ('pal' can be a synonym of 'chum').
'o'+'pal'='OPAL'
'is' acts as a link.
(Other definitions for opal that I've seen before include "Type of gem" , "raw material" , "Lustrous stone" , "precious jewel" , "Semi-precious stone of b;uish white colour" .)