Trainee in the wrong from time to time (6)
I believe the answer is:
intern
'trainee' is the definition.
(intern can mean a trainee worker)
'in the wrong from time to time' is the wordplay.
'from time to time' indicates alternate letters (letters removed on a regular basis).
'the'+'wrong'='thewrong'
The alternate letters of 'thewrong' are 'tern'.
'in'+'tern'='INTERN'
(Other definitions for intern that I've seen before include "Imprison the young doctor without trial" , "cage" , "Houseman" , "Confine - person on work experience" , "Confine - assistant medic in America" .)