In good health in spring (4)
I believe the answer is:
well
This clue is a double definition.
'in good health' is the first definition.
(similar in meaning)
'spring' is the second definition.
(both can mean a source of water)
'in' acts as a link.
(Other definitions for well that I've seen before include "Source of oil - healthy - goodness!" , "31 [SPRING]" , "satisfactory manner?" , "Spring - in good health" , "Abundant source" .)