Going back, see the trainee's mistake (4-2)
I believe the answer is:
slip-up
'mistake' is the definition.
(slipping up is a kind of mistaking)
'going back see the trainee's' is the wordplay.
'going back' is a reversal indicator.
'see the trainee's' becomes 'pupils' (I can't explain this - if you can you should believe this answer much more).
'pupils' in reverse letter order is 'SLIP-UP'.
Can you help me to learn more?
(Other definitions for slip-up that I've seen before include "Minor inadvertent mistake" , "Mistake, blunder" , "Minor lapse" , "Small error" .)