Devious mate's pocketing your old gemstone (8)
I believe the answer is:
amethyst
'gemstone' is the definition.
(amethyst is a kind of gemstone)
'devious mate's pocketing your old' is the wordplay.
'devious' indicates anagramming the letters.
'pocketing' indicates putting letters inside.
'your old' becomes 'thy'.
'mates' anagrammed gives 'amest'.
'amest' enclosing 'thy' is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "Bluish-violet 3" , "Variety of quartz" , "Purple gem" , "Type of quartz" , "Violet-coloured stone" .)