The most common and most likely answer to this crossword clue is the 14 letter word ALL-OR-NOTHING
I've seen this crossword clue in Financial Times
A
L
L
-
O
R
-
N
O
T
H
I
N
G
Buddy explains!
The straight part
We know that "Desperate effort" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case desperate effort = all-or-nothing.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.