Fight prisoner on trial (7)
I believe the answer is:
contest
'fight' is the definition.
(thesaurus)
'prisoner on trial' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'on' is a charade indicator (letters next to each other).
'trial' becomes 'test' (synonyms).
'con'+'test'='CONTEST'
(Other definitions for contest that I've seen before include "Oppose - competition" , "Competition; dispute" , "Tournament" , "Competition or conflict" , "Match" .)