Extremely useful source of water, all right? (4,4)
I believe the answer is:
very well
'all right?' is the definition.
(I know that all right can be written as very well)
'extremely useful source of water' is the wordplay.
'extremely useful' becomes 'very' (I can't justify this - if you can you should believe this answer much more).
'source of water' becomes 'well' (water may come from a well).
'very'+'well'='VERY WELL'
Can you help me to learn more?
(Other definitions for very well that I've seen before include "if you want" , "fine" , "Probably" , "If you insist" , "If you say so - in extremely good health" .)