Challenge cheat on trial (7)
I believe the answer is:
contest
'trial' is the definition.
(thesaurus)
'challenge cheat on' is the wordplay.
'challenge' becomes 'test'.
'cheat' becomes 'con' (con is a kind of cheat).
'on' means one lot of letters go next to another.
'test' after 'con' is 'CONTEST'.
(Other definitions for contest that I've seen before include "Tournament" , "Competition; dispute" , "Oppose - competition" , "Competition or conflict" , "Match" .)