The most common and most likely answer to this crossword clue is the 9 letter word IMPLOSION
I've seen this crossword clue in The Guardian
I
M
P
L
O
S
I
O
N
More clues leading to the result IMPLOSION
Buddy explains!

The straight part
We know that "collapse" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case collapse = implosion.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.
What does IMPLOSION mean?
IMPLOSION noun- The inrush of air in forming a suction stop.
- The action of imploding.
- The act or action of bringing to or as if to a center.