Stays behind regarding water supply (7)
I believe the answer is:
remains
'stays behind' is the definition.
(I've seen this in another clue)
'regarding water supply' is the wordplay.
'regarding' becomes 're' ('re' means regarding or about).
'water supply' becomes 'mains' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
're'+'mains'='REMAINS'
Can you help me to learn more?
(Other definitions for remains that I've seen before include "The body: endures" , "Corpse; relics" , "Residue" , "Leftovers" , "Remnants" .)