The most common and most likely answer to this crossword clue is the 5 plus 2 plus 3 letter word GOING TO POT
I've seen this crossword clue in The Daily Telegraph
G
O
I
N
G
T
O
P
O
T
More clues leading to the result GOING TO POT
Buddy explains!

The straight part
We know that "Deteriorating/becoming a drug-taker (" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deteriorating/becoming a drug-taker ( = going to pot.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.