Trainee's heading off for a profitable job (6)
I believe the answer is:
earner
'a profitable job' is the definition.
'trainee's heading off' is the wordplay.
'trainee' becomes 'learner' (I've seen this before).
'heading off' means to remove the first letter.
'learner' with its first letter taken away is 'EARNER'.
'for' is the link.
(Other definitions for earner that I've seen before include "Profitable undertaking" , "Something bringing in money" , "One bringing in wage" , "way to make money" , "Something generating a good profit" .)