Tyrannical side Copt let loose (8)
I believe the answer is:
despotic
'tyrannical' is the definition.
('despotic' can be a synonym of 'tyrannical')
'side copt let loose' is the wordplay.
'let loose' is an anagram indicator.
'side'+'copt'='sidecopt'
'sidecopt' is an anagram of 'DESPOTIC'.
(Other definitions for despotic that I've seen before include "Authoritarian" , "Dictatorial" , "Like a cruel ruler" , "Tyrannical, oppressive" , "of tyrant" .)