I'm so pure I must change to be tyrannical (9)
I believe the answer is:
imperious
'be tyrannical' is the definition.
(thesaurus)
'i'm so pure i must change' is the wordplay.
'must change' indicates anagramming the letters (I've seen this in other clues).
'so'+'pure'+'i'='sopurei'
'sopurei' with letters rearranged gives 'perious'.
'im'+'perious'='IMPERIOUS'
'to' acts as a link.
(Other definitions for imperious that I've seen before include "Haughty - domineering" , "Bossy" , "high-handed" , "Arrogant, domineering" .)