The most common and most likely answer to this crossword clue is the 2 plus 1 plus 4 plus 3 plus 1 plus 6 letter word ON A WING AND A PRAYER
O
N
A
W
I
N
G
A
N
D
A
P
R
A
Y
E
R
Buddy explains!
The straight part
We know that "unlikely to succeed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unlikely to succeed = on a wing and a prayer.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.