Objection made by expert before trial (7)
I believe the answer is:
protest
'objection' is the definition.
(protest is a kind of objection)
'expert before trial' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'before' means one lot of letters go next to another.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'pro'+'test'='PROTEST'
'made by' is the link.
(Other definitions for protest that I've seen before include "Complain" , "Disapprove" , "Statement or action showing disapproval" , "State objection" , "Express opposition" .)