The most common and most likely answer to this crossword clue is the 12 letter word UNREASONABLE
I've seen this crossword clue in Financial Times
U
N
R
E
A
S
O
N
A
B
L
E
Buddy explains!
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 UNREASONABLE mean?
UNREASONABLE adjective- Without the ability to reason; unreasoning.
- Not reasonable; going beyond what could be expected or asked for.