Bench rabbit who's kept in Test (8)
I believe the answer is:
tribunal
'bench' is the definition.
(thesaurus)
'rabbit who's kept in test' is the wordplay.
'rabbit who' becomes 'bun' (I am not sure about this - if you are sure you should believe this answer much more).
'kept in' indicates putting letters inside.
'test' becomes 'trial' (synonyms).
'bun' placed within 'trial' is 'TRIBUNAL'.
Can you help me to learn more?
(Other definitions for tribunal that I've seen before include "Court of arbitration - unlit bar (anag)" , "examination" , "Assembly to conduct judicial business" , "Adjudicating board" , "A panel of judges" .)