Joker swallows small stone (6)
I believe the answer is:
jasper
'stone' is the definition.
(I know that jasper is a type of opaque gem)
'joker swallows small' is the wordplay.
'joker' becomes 'japer' (I've seen this before).
'swallows' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'japer' going around 's' is 'JASPER'.
(Other definitions for jasper that I've seen before include "hard matter" , "Precious stone - boy's name" , "Semi-precious quartz stone -- boy's name" , "Semi-precious form of quartz" , "Reddish-brown stone used in jewellery" .)