The most common and most likely answer to this crossword clue is the 9 plus 3 letter word UNWRITTEN LAW
I've seen this crossword clue in The Guardian
U
N
W
R
I
T
T
E
N
L
A
W
Buddy explains!
The straight part
We know that "Widely applied code" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case widely applied code = unwritten law.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.