Demonstration in favour of trial (7)
I believe the answer is:
protest
'demonstration' is the definition.
(I know that demonstration 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 dissent" , "Complaint" , "Express an objection" , "Potters (anag.)" , "Demurral" .)