Junior detains our unruly panellist (5)
I believe the answer is:
juror
'panellist' is the definition.
(I know that juror is a type of panellist)
'junior detains our unruly' is the wordplay.
'junior' becomes 'jr' (abbreviation).
'detains' means one lot of letters goes inside another.
'unruly' indicates an anagram.
'our' anagrammed gives 'uro'.
'jr' enclosing 'uro' is 'JUROR'.
(Other definitions for juror that I've seen before include "One in box in court" , "Member of panel in court" , "One of 12 in court" , "Trial panellist" , "One serving in a criminal court" .)