The most common and most likely answer to this crossword clue is the 14 letter word OVER-CONFIDENT
I've seen this crossword clue in The Times
O
V
E
R
-
C
O
N
F
I
D
E
N
T
Buddy explains!
The straight part
We know that "Too sure of oneself" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case too sure of oneself = over-confident.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.