Canny trial turned despotic (10)
I believe the answer is:
tyrannical
'despotic' is the definition.
(I know that despotic can be written as tyrannical)
'canny trial turned' is the wordplay.
'turned' indicates an anagram.
'canny'+'trial'='cannytrial'
'cannytrial' is an anagram of 'TYRANNICAL'.
(Other definitions for tyrannical that I've seen before include "Unjustly oppressive" , "Autocratic" .)