The most common and most likely answer to this crossword clue is the 7 letter word MANHOLE
I've seen this crossword clue in Independent
M
A
N
H
O
L
E
Buddy explains!
The straight part
We know that "access to water supply?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case access to water supply? = manhole.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 MANHOLE mean?
MANHOLE noun- A hole in the ground used to access the sewers or other underground vaults and installations.
- A hole providing access to the inside of a boiler, tank etc.
- A man's anus, in a sexual context.