Objection in favour of trial (7)
I believe the answer is:
protest
'objection' is the definition.
(I know that objection can be written as protest)
'in favour of trial' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'trial' becomes 'test' (synonyms).
'pro'+'test'='PROTEST'
(Other definitions for protest that I've seen before include "Express opposition" , "Record objection" , "Object strenuously" , "Public demonstration" , "Disapprove" .)