The most common and most likely answer to this crossword clue is the 12 letter word PRESUMPTUOUS
I've seen this crossword clue in The Daily Telegraph
P
R
E
S
U
M
P
T
U
O
U
S
More clues leading to the result PRESUMPTUOUS
Buddy explains!

The straight part
We know that "Assuming" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assuming = presumptuous.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 PRESUMPTUOUS mean?
PRESUMPTUOUS adjective- Going beyond what is right, proper, or appropriate because of an excess of self-confidence or arrogance.