The most common and most likely answer to this crossword clue is the 8 letter word REBELLED
I've seen this crossword clue in The Daily Telegraph
R
E
B
E
L
L
E
D
More clues leading to the result REBELLED
Buddy explains!

The straight part
We know that "broke with established customs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case broke with established customs = rebelled.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.
What does REBELLED mean?
REBELLED verb- To resist or become defiant toward an authority.