Demonstrations in favour of trials (8)
I believe the answer is:
protests
'demonstrations' is the definition.
(I've seen this before)
'in favour of trials' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'trials' becomes 'tests' ('test' can be a synonym of 'trial').
'pro'+'tests'='PROTESTS'
(Other definitions for protests that I've seen before include "Raises objection" , "Expresses an objection" , "Public demonstrations to object to something" , "Complaints and objections" .)