Widely applied code that can't be read (9,3)
I believe the answer is:
unwritten law
'read' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'widely applied code that can't' is the wordplay.
I cannot really understand how this works, but
'code' could be 'law' (legal code) and 'law' is present in the answer.
The remaining letters 'unwritten' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'be' acts as a link.
Can you help me to learn more?
(Another definition for unwritten law that I've seen is " Accepted rule".)