The most common and most likely answer to this crossword clue is the 1 plus 7 letter word A HANDBAG
I've seen this crossword clue in Financial Times
A
H
A
N
D
B
A
G
Buddy explains!
The straight part
We know that "25, 27’s unacceptable breeding environment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 25, 27’s unacceptable breeding environment = a handbag.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.