Dictator's extremely testy diatribe (6)
I believe the answer is:
tyrant
'dictator's' is the definition.
(I have seen 'Cruel dictator ' mean 'tyrant' so perhaps 'dictator' could also mean 'tyrant')
'extremely testy diatribe' is the wordplay.
'extremely' means to remove the middle letters.
'diatribe' becomes 'rant' (I've seen this in other clues).
'testy' with its middle removed is 'ty'.
'ty'+'rant'='TYRANT'
(Other definitions for tyrant that I've seen before include "Arbitrary ruler" , "Person dictating" , "Cruel and oppressive dictator" , "Despot, dictator" , "Totalitarian" .)