The most common and most likely answer to this crossword clue is the 15 letter word UNPARLIAMENTARY
I've seen this crossword clue in The Times
U
N
P
A
R
L
I
A
M
E
N
T
A
R
Y
More clues leading to the result UNPARLIAMENTARY
Buddy explains!

The straight part
We know that "this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this = unparliamentary.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.
What does UNPARLIAMENTARY mean?
UNPARLIAMENTARY adjective- Unsuitable to be used in parliament
- Contrary to the rules of parliament