Oppressor’s extremely tacky tirade (6)
I believe the answer is:
tyrant
'oppressor's' is the definition.
(thesaurus)
'extremely tacky tirade' is the wordplay.
'extremely' means to remove the middle letters.
'tirade' becomes 'rant' (I've seen this before).
'tacky' with its centre taken out is 'ty'.
'ty'+'rant'='TYRANT'
(Other definitions for tyrant that I've seen before include "Person dictating" , "Despotic ruler" , "Autocrat - bully" , "bully - with 17d" , "big bully" .)