Select former unit on hospital department (9)
I believe the answer is:
excellent
'select' is the definition.
(thesaurus)
'former unit on hospital department' is the wordplay.
'former' becomes 'ex'.
'unit' becomes 'cell' (cell is a kind of unit**).
'on' means one lot of letters go next to another.
'hospital department' becomes 'ent'.
'ex'+'cell'+'ent'='EXCELLENT'
(Other definitions for excellent that I've seen before include "capital" , "Great!" , "brilliant" , "Quite splendid, extremely good" , "Really good, top class" .)