Embarrassing mistake about dull, back-pedalling, perennial protesters? (4-1-3)
I believe the answer is:
rent-a-mob
'perennial protesters?' is the definition.
(disparaging term for protestors)
'embarrassing mistake about dull back-pedalling' is the wordplay.
'embarrassing mistake' becomes 'boner' (informal term for a blunder).
'about' means one lot of letters goes inside another.
'dull' becomes 'mat' (mat as opposed to glossy).
'back-pedalling' is a reversal indicator.
'boner' placed around 'mat' is 'bomatner'.
'bomatner' reversed gives 'RENT-A-MOB'.
(Other definitions for rent-a-mob that I've seen before include "mass of people" , "Crowd for hire" , "Hired crowd (facet.)" .)