One properly sunk (4)
I believe the answer is:
well
This might be a double definition.
'one properly' is the first definition.
(thesaurus)
'sunk' is the second definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for well that I've seen before include "rightly" , "Sunken shaft; very much" , "Healthy; extremely" , "Spring - in good condition" , "water is found here" .)