The most common and most likely answer to this crossword clue is the 9 letter word REGRESSED
I've seen this crossword clue in The Daily Telegraph
R
E
G
R
E
S
S
E
D
Buddy explains!
The straight part
We know that "deteriorated" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deteriorated = regressed.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 REGRESSED mean?
REGRESSED verb- To move backwards to an earlier stage; to devolve.
- To move from east to west.
- To perform a regression on an explanatory variable.
- That has been subject to regression