The most common and most likely answer to this crossword clue is the 12 letter word DISINTEGRATE
I've seen this crossword clue in The Times
D
I
S
I
N
T
E
G
R
A
T
E
More clues leading to the result DISINTEGRATE
- Cooked up strange diet including one crumble
- Smash dish in the fireplace — not hard at all
- Crumble made with fruit? Ties in when cooked with grain inside
- I get strained terribly and fall apart
- Titian’s greed stupidly causing break up
- Unexpectedly ingest dirt with crust of apple crumble
- Smash and dig up bottomless jar
- I must enter complicated reading test, but go to pieces
- Around top of grave unearth corroded decay
- Break up and scatter inside target
Buddy explains!

The straight part
We know that "crumble" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case crumble = disintegrate.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 DISINTEGRATE mean?
DISINTEGRATE verb- To undo the integrity of, break into parts.
- To fall apart, break up into parts.