The most common and most likely answer to this crossword clue is the 4 plus 4 plus 7 plus 4 letter word WHEN HELL FREEZES OVER
I've seen this crossword clue in The Guardian
W
H
E
N
H
E
L
L
F
R
E
E
Z
E
S
O
V
E
R
Buddy explains!
The straight part
We know that "Never!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case never! = when hell freezes over.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.