The most common and most likely answer to this crossword clue is the 7 letter word BUCKLED
I've seen this crossword clue in The Daily Telegraph
B
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K
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More clues leading to the result BUCKLED
Buddy explains!

The straight part
We know that "collapsed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case collapsed = buckled.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 BUCKLED mean?
BUCKLED verb- To distort or collapse under physical pressure; especially, of a slender structure in compression.
- To make bend; to cause to become distorted.
- To give in; to react suddenly or adversely to stress or pressure (of a person).
- To yield; to give way; to cease opposing.
- To enter upon some labour or contest; to join in close fight; to contend.
- To buckle down; to apply oneself.
- To fasten using a buckle.
- To unite in marriage.