Upset, trainees do it badly (12)
I believe the answer is:
disorientate
'upset' is the definition.
(both can mean to confuse or disturb)
'trainees do it badly' is the wordplay.
'badly' indicates anagramming the letters.
'trainees'+'do'+'it'='traineesdoit'
'traineesdoit' with letters rearranged gives 'DISORIENTATE'.
(Other definitions for disorientate that I've seen before include "Cause to lose a sense of direction" , "Cause to feel confused" , "puzzle?" , "Perplex" , "Muddle" .)