Test involving roll bar (8)
I believe the answer is:
tribunal
'bar' is the definition.
(thesaurus)
'test involving roll' is the wordplay.
'test' becomes 'trial' ('trial' can be a synonym of 'test').
'involving' means one lot of letters goes inside another.
'roll' becomes 'bun' ('bun' can be a synonym of 'roll').
'trial' going around 'bun' is 'TRIBUNAL'.
(Other definitions for tribunal that I've seen before include "Adjudicating board" , "Hearing" , "Court of justice or arbitration" , "Court, investigative board" , "Judgement seat" .)