Poll tax protester rival to Thatcher mentioned? (5)
I believe the answer is:
tyler
'poll tax protester' is the definition.
(Wat Tyler, leader of the Peasants' Revolt)
'rival to thatcher mentioned?' is the wordplay.
'rival to thatcher' becomes 'tiler' (someone who tiles roofs rather than thatching them).
'mentioned?' shows a homophone (sound like) (I've seen 'mentioned' mean this).
'tiler' sounds like 'TYLER'.
(Other definitions for tyler that I've seen before include "He had responsibility for rising" , "Revolting man" , "Revolutionary" , "Wat, leader of the Peasants' Revolt" .)