Don't interfere - with the water supply? (3,4,5)
I believe the answer is:
let well alone
'supply?' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'don't interfere with the water' is the wordplay.
'don't interfere' becomes 'letalone' (I can't justify this - if you can you should believe this answer much more).
'with' indicates putting letters inside.
'the water' becomes 'well' (well water is a kind of water).
'letalone' enclosing 'well' is 'LET WELL ALONE'.
Can you help me to learn more?
(Other definitions for let well alone that I've seen before include "Don't stir up potential trouble" , "Refrain from trying to improve what is already satisfactory" , "OK as it is" , "Desist from action for fear of making worse" , "Refrain from interfering with satisfactory situation" .)