A state in Eastern US, in short, upset another in East (5)
I believe the answer is:
assam
'a state' is the definition.
(I've seen this before)
'eastern us in short upset another in east' is the wordplay.
'eastern' shows that the letters should be reversed in order (letters written right to left or east to west).
'us' becomes 'man' (mankind).
'in short' means to remove the last letter.
'upset another in east' becomes 'ssa' (I can't justify this - if you can you should believe this answer much more).
'man' with its last letter taken away is 'ma'.
'ma'+'ssa'='massa'
'massa' reversed gives 'ASSAM'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for assam that I've seen before include "Kind of Indian tea" , "Indian state known for its tea production" , "Tea-growing region" , "Tea-growing state in north-east India" , "Indian tea growing area" .)