The most common and most likely answer to this crossword clue is the 9 letter word UNMETERED
I've seen this crossword clue in The Times
U
N
M
E
T
E
R
E
D
Buddy explains!
The straight part
We know that "Sort of water supply perhaps" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sort of water supply perhaps = unmetered.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 UNMETERED mean?
UNMETERED adjective- Not metered, not charged on a meter (as Internet bandwidth).