The most common and most likely answer to this crossword clue is the 5 plus 12 letter word CIVIL DISOBEDIENCE
C
I
V
I
L
D
I
S
O
B
E
D
I
E
N
C
E
Buddy explains!
The straight part
We know that "peaceful form of protest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case peaceful form of protest = civil disobedience.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.