Small, the old man in health resort (3)
I believe the answer is:
spa
'health resort' is the definition.
(thesaurus)
'small the old man' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'the old man' becomes 'pa' ('pa' can be a synonym of 'old man'**).
's'+'pa'='SPA'
'in' acts as a link.
(Other definitions for spa that I've seen before include "town for his health!" , "Bath - mineral water spring" , "Hydromassage facility" , "Bath, possibly" , "eg Vichy" .)