Boxed in, one has a trying time (5)
I believe the answer is:
juror
'one has a trying time' is the definition.
(a juror participates in a trial)
I don't understand how the rest of the clue works.
(Other definitions for juror that I've seen before include "Court panellist" , "One serving in a criminal court" , "Member of panel in court" , "One of 12 in court" , "Trial panellist" .)