Old Spanish silver, gallery's property (4,6)
I believe the answer is:
real estate
'property' is the definition.
(real estate is a kind of property)
'old spanish silver gallery's' is the wordplay.
'old spanish silver' becomes 'reales' (the real was a historical silver coin of Spain).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'reales'+'tate'='REAL ESTATE'
(Other definitions for real estate that I've seen before include "Land in America" , "properties" , "Landed property" , "land in New England" , "Land (mainly US)" .)