The most common and most likely answer to this crossword clue is the 12 letter word PREPOSTEROUS
I've seen this crossword clue in Financial Times
P
R
E
P
O
S
T
E
R
O
U
S
More clues leading to the result PREPOSTEROUS
- Utterly absurd new supporter, so excited initially
- Before Mailer, nothing American is unbelievable
- Wretched poseur holding placard that’s absurd
- Outrageous advert in tortured prose outside university
- Absurd, our sprees – opt to change
- Absurd potty American admits dealing with Bill
- Ridiculous trousers start to expand, and pop undone
- Pope with trousers off – that’s unthinkable!
- Absurd purpose store devised
- Trousers pop undone around end of farce? Farcical
Buddy explains!

The straight part
We know that "Absurd" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case absurd = preposterous.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 PREPOSTEROUS mean?
PREPOSTEROUS adjective- Absurd, or contrary to common sense.