Boss on hospital department’s pupils (8)
I believe the answer is:
students
'pupils' is the definition.
('student' can be a synonym of 'pupil')
'boss on hospital department's' is the wordplay.
'boss' becomes 'stud' (synonyms).
'on' says to put letters next to each other.
'hospital department's' becomes 'ents'.
'stud'+'ents'='STUDENTS'
(Other definitions for students that I've seen before include "Trainee medics possibly" , "Learners or scholars" , "Sent dust to those who learn" , "Teachers teach them" , "Pupils at school, university etc" .)