Having taken the water cure successfully? (4)
I believe the answer is:
well
'successfully?' is the definition.
(I've seen this in other clues)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for well that I've seen before include "Spring - in good condition" , "Very; without difficulty; bore" , "Sound" , "Pour - thoroughly" , "and 24: Almost" .)