Bosses welcoming hospital department's trainees (8)
I believe the answer is:
students
'trainees' is the definition.
'bosses welcoming hospital department's' is the wordplay.
'bosses' becomes 'studs' (boss can mean a type of metal stud).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'hospital department' becomes 'ENT' (abbreviation for Ear, Nose and Throat department).
'studs' placed around 'ent' is 'STUDENTS'.
(Other definitions for students that I've seen before include "Learners as at school" , "Pupils or scholars" , "Learners or scholars" , "Eg, undergraduates" , "Pupils or apprentices" .)