Case in hospital not so young (6)
I believe the answer is:
holder
'case' is the definition.
(thesaurus)
'hospital not so young' is the wordplay.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'not so young' becomes 'older' ('old' can be the opposite of 'young' so perhaps 'older' is reasonable for 'not so young').
'h'+'older'='HOLDER'
'in' is the link.
(Other definitions for holder that I've seen before include "Something which grasps" , "Occupant" , "Keeper; container" , "retainer" , "Noddy" .)