The most common and most likely answer to this crossword clue is the 6 letter word DOVISH
I've seen this crossword clue in The Sunday Telegraph
D
O
V
I
S
H
More clues leading to the result DOVISH
Buddy explains!
The straight part
We know that "Party opposing one call for peace is not this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case party opposing one call for peace is not this = dovish.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.