In the Orient, Society that is most casual (7)
I believe the answer is:
easiest
'most casual' is the definition.
('easy' can be similar in meaning to 'casual')
'in the orient society that is' is the wordplay.
'in the' means one lot of letters goes inside another.
'orient' becomes 'east' ('East' can be a synonym of 'orient').
'society' becomes 's' (common abbreviation in names of organisations).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
's'+'ie'='sie'
'east' placed around 'sie' is 'EASIEST'.
(Other definitions for easiest that I've seen before include "Least difficult" , "Requiring the least amount of effort" , "Simplest" , "Least hard" , "Most comfortable" .)