The most common and most likely answer to this crossword clue is the 6 letter word TOCSIN
I've seen this crossword clue in The Guardian
T
O
C
S
I
N
Buddy explains!

The straight part
We know that "Bell rung" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bell rung = tocsin.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 TOCSIN mean?
TOCSIN noun- An alarm or other signal sounded by a bell or bells, originally especially with reference to France.
- A bell used to sound an alarm.