Possibly five hundred needed to make a castle? (4)
I believe the answer is:
sand
'make a castle?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'possibly five hundred needed' is the wordplay.
'possibly' becomes 'san' (I can't justify this - if you can you should believe this answer much more).
'five hundred' becomes 'D' (Roman numeral).
'needed' is a charade indicator (letters next to each other).
'san'+'d'='SAND'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for sand that I've seen before include "Beach/desert material" , "yellowish-brown" , "Beach constituent" , "Smooth with abrasive paper" , "Rock particles; smooth" .)