The most common and most likely answer to this crossword clue is the 7 plus 4 letter word HORNETS NEST
I've seen this crossword clue in The Sunday Telegraph
H
O
R
N
E
T
S
N
E
S
T
Buddy explains!
The straight part
We know that "— better not stir it up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case — better not stir it up = hornets nest.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.