Dominant eastern state (5)
I believe the answer is:
maine
'state' is the definition.
(US state)
'dominant eastern' is the wordplay.
'dominant' becomes 'main' (I've seen this before).
'eastern' becomes 'e'.
'main'+'e'='MAINE'
(Other definitions for maine that I've seen before include "American state, capital Augusta" , "somewhere in America" , "North-east American state" , "New England state" , "N. US Atlantic state" .)