Trainee left a breadwinner (7)
I believe the answer is:
learner
'trainee' is the definition.
(I've seen this before)
'left a breadwinner' is the wordplay.
'left' becomes 'l' (common abbreviation).
'a breadwinner' becomes 'earner' (I've seen this before).
'l'+'earner'='LEARNER'
(Other definitions for learner that I've seen before include "One ascertaining" , "who needs plates" , "One being taught" , "Pupil or apprentice" , "One gaining knowledge" .)