Trainee boss on hospital department (7)
I believe the answer is:
student
'trainee' is the definition.
(I've seen this before)
'boss on hospital department' is the wordplay.
'boss' becomes 'stud' ('stud' can be a synonym of 'boss').
'on' is a charade indicator (letters next to each other).
'hospital department' becomes 'ent'.
'stud'+'ent'='STUDENT'
(Other definitions for student that I've seen before include "University attendee" , "Person engaged in learning" , "Scholar" , "collegian" , "One at university" .)