Stone some castles may be built of (5)
I believe the answer is:
slate
'be built of' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'stone some castles' is the wordplay.
'stone' is an anagram indicator (I've seen 'stoned' mean this (can mean on drugs or drunk)).
'some' means to look at the middle letters.
The middle letters of 'castles' are 'astle'.
'astle' with letters rearranged gives 'SLATE'.
'may' is the link.
Can you help me to learn more?
(Other definitions for slate that I've seen before include "Metamorphic rock that can be split into thin layers" , "Roofing tile" , "Roofing plate" , "Rubbish" , "Scold; hard rock" .)