Joining hospital not as new case (6)
I believe the answer is:
holder
'case' is the definition.
(thesaurus)
'joining hospital not as new' is the wordplay.
'joining' means one lot of letters go next to another.
'hospital' becomes 'h'.
'not as new' becomes 'older' ('old' can be the opposite of 'new' so perhaps 'older' is reasonable for 'not as new').
'h' next to 'older' is 'HOLDER'.
(Other definitions for holder that I've seen before include "Keeper; container" , "retainer" , "Something which grasps" , "Occupant" , "Noddy" .)