Demonstrate expert trial (7)
I believe the answer is:
protest
'demonstrate' is the definition.
(I know that demonstrate is a more specific form of the action protest)
'expert trial' is the wordplay.
'expert' becomes 'pro' (short for professional).
'trial' becomes 'test' (synonyms).
'pro'+'test'='PROTEST'
(Other definitions for protest that I've seen before include "Demurral" , "Object strenuously" , "Record objection" , "Take exception" , "Complaint" .)