The most common and most likely answer to this crossword clue is the 13 letter word MIND-BOGGLING
I've seen this crossword clue in The Times
M
I
N
D
-
B
O
G
G
L
I
N
G
Buddy explains!
The straight part
We know that "extraordinary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extraordinary = mind-boggling.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 MIND-BOGGLING mean?
MIND-BOGGLING adjective- That causes the mind to boggle; that is beyond one's ability to understand or figure out.