Royal castle (6)
(Other definitions for palace that I've seen before include "Stately mansion" , "football team" , "Locality round a monarch's official residence" , "Large building" , "Royal house" .)
'royal castle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for windsor that I've seen before include "Barbara" , "town near Eton" , "Berkshire castle" , "Royal palace" , "Type of soap" .)