First in sauna, the old man in health resort (3)
I believe the answer is:
spa
'health resort' is the definition.
(I've seen this before)
'first in sauna the old man' is the wordplay.
'first in sauna' becomes 's' (pst letter of 'sauna').
'the old man' becomes 'pa' (both can mean father).
's'+'pa'='SPA'
'in' is the link.
(Other definitions for spa that I've seen before include "Curative resort" , "Bath - mineral water spring" , "here for healthy holiday" , "here for the good of his health?" , "Watering hole" .)