A word about castles by the sea (4)
I believe the answer is:
sand
'castles by the sea' is the definition.
I can't tell whether this defines the answer.
'a word about' is the wordplay.
'a' becomes 'an'.
'word' becomes 'sd' (I am not sure about this - if you are sure you should believe this answer much more).
'about' means one lot of letters goes inside another.
'an' inserted within 'sd' is 'SAND'.
Can you help me to learn more?
(Other definitions for sand that I've seen before include "Fine stone particles" , "Loose grains" , "George -, nineteenth century French novelist" , "Desert material" , "It's used in glass-making" .)