Boss of former unit on hospital department (9)
I believe the answer is:
excellent
'boss' is the definition.
(boss can mean very good)
'former unit on hospital department' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'unit' becomes 'cell' (cell is a kind of unit).
'on' says to put letters next to each other.
'hospital department' becomes 'ENT' (abbreviation for Ear, Nose and Throat department).
'ex'+'cell'+'ent'='EXCELLENT'
'of' acts as a link.
(Other definitions for excellent that I've seen before include "capital" , "First-class" , "Great!" , "Admirable, first-rate" , "Really good, top class" .)