The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 letter word SINK OR SWIM
I've seen this crossword clue in The Daily Telegraph
S
I
N
K
O
R
S
W
I
M
More clues leading to the result SINK OR SWIM
Buddy explains!

The straight part
We know that "If you don’t succeed, you’ll fail" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case if you don’t succeed, you’ll fail = sink or swim.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.