Demo involving professional before trial (7)
I believe the answer is:
protest
'demo' is the definition.
'professional before trial' is the wordplay.
'professional' becomes 'pro' (abbreviation).
'before' is a charade indicator (letters next to each other).
'trial' becomes 'test' (synonyms).
'pro'+'test'='PROTEST'
'involving' is the link.
(Other definitions for protest that I've seen before include "Demurral" , "Public demonstration" , "Express disagreement" , "Express an objection" , "Potters (anag.)" .)