Such castles are prone to flooding (4)
I believe the answer is:
sand
'such castles are' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sand that I've seen before include "Yellow-brown colour" , "Rock particles; smooth" , "George -, French author" , "lover of Chopin" , "Very fine grains of rock, as on beach" .)