Specialist trial demo (7)
I believe the answer is:
protest
'demo' is the definition.
'specialist trial' is the wordplay.
'specialist' becomes 'pro' (I've seen this in another clue).
'trial' becomes 'test' (synonyms).
'pro'+'test'='PROTEST'
(Other definitions for protest that I've seen before include "State objection" , "Disapprove" , "Record objection" , "Take exception" , "Express opposition" .)