Part of US, old habitat Indians occupied, originally (4)
I believe the answer is:
ohio
'part of us' is the definition.
('Ohio' is part of 'us')
'old habitat indians occupied originally' is the wordplay.
'originally' says to take the initial letters.
The first letters of 'old habitat indians occupied' is 'OHIO'.
(Other definitions for ohio that I've seen before include "Midwestern US state and river" , "state of America" , "great river" , "The Buckeye State" , "E. central US state" .)