Experimenter and demonstrator dismissing expert (6)
I believe the answer is:
tester
'experimenter' is the definition.
(thesaurus)
'demonstrator dismissing expert' is the wordplay.
'demonstrator' becomes 'protester' (synonyms).
'dismissing' is a deletion indicator.
'expert' becomes 'pro' (short for professional).
'protester' with 'pro' taken away is 'TESTER'.
'and' acts as a link.
(Other definitions for tester that I've seen before include "One trying out; canopy over bed" , "old coin" , "One who analyses a bed canopy" , "Examiner - canopy over a four-poster" , "covering at night" .)