Criticise one hollow protester (9)
I believe the answer is:
dissident
'protester' is the definition.
(I know that protester can be written as dissident)
'criticise one hollow' is the wordplay.
'criticise' becomes 'diss' (to diss is to criticise or insult).
'one' becomes 'I' (Roman numeral).
'hollow' becomes 'dent' (both can mean an impression or indentation).
'diss'+'i'+'dent'='DISSIDENT'
(Other definitions for dissident that I've seen before include "One opposed to official policy" , "Protester" , "Person opposing official policy" , "Political nonconformist" , "Opposer of officialdom" .)