The most common and most likely answer to this crossword clue is the 9 letter word CRIPPLING
I've seen this crossword clue in The Times
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More clues leading to the result CRIPPLING
Buddy explains!

The straight part
We know that "disastrous undermining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case disastrous undermining = crippling.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 CRIPPLING mean?
CRIPPLING verb- To make someone a cripple; to cause someone to become physically impaired
- To damage seriously; to destroy
- To release a product (especially a computer program) with reduced functionality, in some cases, making the item essentially worthless.
- To nerf something which is overpowered
- State of being crippled; lameness.
- Spars or timbers set up as a support against the side of a building.
- That cripples or incapacitates