It’s usually kept half walled up (3)
I believe the answer is:
law
'it's usually kept' is the definition.
I can't tell whether this definition defines the answer.
'half walled up' is the wordplay.
'half' means to take half the letters.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'walled' cut in half is 'wal'.
'wal' reversed gives 'LAW'.
Can you help me to learn more?
(Other definitions for law that I've seen before include "Edict" , "Rule prohibiting certain actions" , "Collection of rules for a society" , "'Possession is nine points of the ...'" , "Rule, statute" .)