The most common and most likely answer to this crossword clue is the 6 plus 4 plus 5 letter word FAMOUS LAST WORDS
I've seen this crossword clue in The Guardian
F
A
M
O
U
S
L
A
S
T
W
O
R
D
S
More clues leading to the result FAMOUS LAST WORDS
- Speaking too soon, American entering incorrect format, Sod's law!
- Say “Kiss me, Hardy”? Don’t tempt fate!
- Et tu Brute? How wrong I was!
- Death sentence widely publicised?
- Overly-optimistic statement, passing comment?
- Star’s awful moods could produce ironic comment
- Something memorable said in passing?
- Wit’s formula, so sad, spouted with one passing away?
- Something memorable said on the way out?
Buddy explains!
The straight part
We know that "Et tu Brute?/How wrong I was!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case et tu brute?/how wrong i was! = famous last words.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.