Some upsetting Thermopylae dictator in thought (4)
I believe the answer is:
idea
'thought' is the definition.
('idea' can be a synonym of 'thought')
'some upsetting thermopylae dictator' is the wordplay.
'upsetting' shows that the letters should be reversed in order.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'aedi' is hidden within 'thermopylae dictator'.
'aedi' back-to-front is 'IDEA'.
'in' acts as a link.
(Other definitions for idea that I've seen before include "Thought, belief" , "Clue" , "Aim" , "It comes to mind" , "intellectual product" .)