Horrid, that which Wat Tyler was doing? (9)
I believe the answer is:
revolting
I believe this clue is a double definition.
'horrid' is the first definition.
(thesaurus)
'that which wat tyler was doing?' is the second definition.
(Wat Tyler led the Peasants' Revolt)
(Other definitions for revolting that I've seen before include "rioting" , "Rising in rebellion" , "Loathsome" , "Repulsive" , "Disgusting" .)