Son and father in health resort (3)
I believe the answer is:
spa
'health resort' is the definition.
(spa can mean a health resort)
'son and father' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'and' means one lot of letters go next to another.
'father' becomes 'pa' (pa can mean dad or father).
's'+'pa'='SPA'
'in' acts as a link.
(Other definitions for spa that I've seen before include "Hydromassage facility" , "hydro" , "town in Belgium" , "well" , "Cheltenham or Bath?" .)