Descriptive of castles by the sea (4)
I believe the answer is:
sand
'sea' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for sand that I've seen before include "George -, Fr. author" , "Smooth down (surface)" , "Grit; powdered rock" , "Sort of paper, piper, castle or martin" , "Granulated quartz" .)