Can try a nil design that’s autocratic (10)
I believe the answer is:
tyrannical
'that's autocratic' is the definition.
(synonyms)
'can try a nil design' is the wordplay.
'design' indicates anagramming the letters.
'can'+'try'+'a'+'nil'='cantryanil'
'cantryanil' anagrammed gives 'TYRANNICAL'.
(Other definitions for tyrannical that I've seen before include "Autocratic" , "Unjustly oppressive" .)