Work on mock trial (5,3)
I believe the answer is:
dummy run
'trial' is the definition.
(I've seen this before)
'work on mock' is the wordplay.
'work' becomes 'run' (both can mean to operate something).
'on' is a charade indicator (letters next to each other).
'mock' becomes 'dummy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'run' put after 'dummy' is 'DUMMY RUN'.
Can you help me to learn more?
(Other definitions for dummy run that I've seen before include "Rehearsal" , "Practice session" , "Practice, trial" , "A try-out or test" .)