Objection on behalf of trial (7)
I believe the answer is:
protest
'objection' is the definition.
(protest is a kind of objection)
'on behalf of trial' is the wordplay.
'on behalf of' becomes 'pro'.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'pro'+'test'='PROTEST'
(Other definitions for protest that I've seen before include "Potters (anag.)" , "Objection from potters" , "Demonstrate" , "Disapprove" , "Spotter of objection" .)