In good health? Fancy that! (4)
I believe the answer is:
well
I believe this is a double definition.
'in good health?' is the first definition.
('well' can be similar in meaning to 'in good health')
'fancy that' is the second definition.
(both are exclamations of surprise)
(Other definitions for well that I've seen before include "Pour - thoroughly" , "31 [SPRING]" , "Facial hair" , "Water - thoroughly" , "Spring - in good condition" .)