Aristotle so thought to contain kingdom (7)
I believe the answer is:
lesotho
'kingdom' is the definition.
(I've seen this before)
'aristotle so thought to contain' is the wordplay.
'to contain' indicates a hidden word (words from the clue contain the answer).
'LESOTHO' is hidden in the letters of 'aristotle so thought'.
(Other definitions for lesotho that I've seen before include "Landlocked country in southern Africa" , "He loots (anag) -- southern African kingdom" , "Kingdom that was formerly Basutoland" , "South African kingdom" , "African country, formerly Basutoland" .)