Trainee in hospital department following boss (7)
I believe the answer is:
student
'trainee' is the definition.
(I've seen this before)
'hospital department following boss' is the wordplay.
'hospital department' becomes 'ent'.
'following' is a charade indicator (letters next to each other).
'boss' becomes 'stud' (synonyms).
'ent' after 'stud' is 'STUDENT'.
'in' is the link.
(Other definitions for student that I've seen before include "Learner, pupil" , "University attendee" , "Maybe fresher" , "Eg, undergraduate" , "collegian" .)