Water source thoroughly (4)
I believe the answer is:
well
I believe this clue is a double definition.
'water source' is the first definition.
(I've seen this before)
'thoroughly' is the second definition.
(I've seen this before)
(Other definitions for well that I've seen before include "Borehole" , "Source; in good health" , "Competently" , "Bore; very much" , "rise up" .)