American Society occupying congress area in SE England (6)
I believe the answer is:
sussex
'area in se england' is the definition.
(county in the south east of England)
'american society occupying congress' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'society' becomes 's' (common abbreviation in names of organisations).
'occupying' means one lot of letters goes inside another.
'congress' becomes 'sex'.
'us'+'s'='uss'
'uss' inserted within 'sex' is 'SUSSEX'.
(Other definitions for sussex that I've seen before include "Tough; like tendons" , "Southern UK county" , "County on the English channel" , "part of southern England" , "South coast county" .)