Water thoroughly (4)
I believe the answer is:
well
This clue is a double definition.
'water' is the first definition.
(well water is a kind of water)
'thoroughly' is the second definition.
(I've seen this before)
(Other definitions for well that I've seen before include "rightly" , "Abundant source" , "Very; without difficulty; bore" , "all right" , "Healthy - water supply" .)