The most common and most likely answer to this crossword clue is the 5 plus 4 plus 5 letter word LEAVE WELL ALONE
I've seen this crossword clue in Financial Times
L
E
A
V
E
W
E
L
L
A
L
O
N
E
More clues leading to the result LEAVE WELL ALONE
Buddy explains!
The straight part
We know that "don’t touch the water supply" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case don’t touch the water supply = leave well alone.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.