China topped by old gem (4)
I believe the answer is:
opal
'gem' is the definition.
(opal is a kind of gem)
'china topped by old' is the wordplay.
'china' becomes 'pal' (I've seen this before).
'topped by' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pal' put after 'o' is 'OPAL'.
(Other definitions for opal that I've seen before include "Form of hydrated silica, often used as a gemstone" , "Semi-precious stone of milky or bluish colour" , "it may be highly valued" , "Delicately-coloured gem" , "Precious stuff" .)