The most common and most likely answer to this crossword clue is the 11 letter word NONVIOLENCE
I've seen this crossword clue in The Guardian
N
O
N
V
I
O
L
E
N
C
E
Buddy explains!
The straight part
We know that "pacifism" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pacifism = nonviolence.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 NONVIOLENCE mean?
NONVIOLENCE noun- A philosophy that rejects the use of violence, and instead seeks to bring about change through peaceful responses even to violent acts.