Before magistrate, prepared for trouble (5)
I believe the answer is:
upset
'for trouble' is the definition.
(upset can mean trouble or controversy)
'before magistrate prepared' is the wordplay.
'before magistrate' becomes 'up' (up in court).
'prepared' becomes 'set' (set up or ready).
'up'+'set'='UPSET'
(Other definitions for upset that I've seen before include "Upheaval" , "Capsize" , "Disturb or knock over" , "Overturn - perturbed - disordered" , "Make unhappy; disrupt" .)