Lock up trainee (6)
I believe the answer is:
intern
This clue is a double definition.
'lock up' is the first definition.
(thesaurus)
'trainee' is the second definition.
(intern can mean a trainee worker)
(Other definitions for intern that I've seen before include "Hospital worker" , "Imprison (enemy aliens)" , "Imprison the young doctor without trial" , "Shut up" , "Detain in custody" .)