The most common and most likely answer to this crossword clue is the 6 plus 2 plus 7 letter word MASQUE OF ANARCHY
I've seen this crossword clue in The Guardian
M
A
S
Q
U
E
O
F
A
N
A
R
C
H
Y
Buddy explains!
The straight part
We know that "25’s response to Peterloo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 25’s response to peterloo = masque of anarchy.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.