Bosses recruit European theatre trainees (8)
I believe the answer is:
students
'trainees' is the definition.
'bosses recruit european theatre' is the wordplay.
'bosses' becomes 'studs' (boss can mean a type of metal stud).
'recruit' means one lot of letters goes inside another (in the sense of bringing in).
'european' becomes 'e' (abbreviation e.g. EU).
'theatre' becomes 'nt' (National Theatre).
'e'+'nt'='ent'
'studs' enclosing 'ent' is 'STUDENTS'.
(Other definitions for students that I've seen before include "School or university attendees" , "Pupils, learners" , "Trainee medics possibly" , "Pupils or scholars" , "Pupils at school, university etc" .)