Son in good health? That’s dandy (5)
I believe the answer is:
swell
'that's dandy' is the definition.
(both can informally mean excellent or satisfactory)
'son in good health?' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in good health?' becomes 'well' (similar in meaning).
's'+'well'='SWELL'
(Other definitions for swell that I've seen before include "Bulge; a toff" , "Balloon - dandy" , "Enlarge; excellent" , "Become larger like toff" , "Movement of the surface of the sea" .)