America's eastern purpose (3)
I believe the answer is:
use
'purpose' is the definition.
('use' can be a synonym of 'purpose')
'america's eastern' is the wordplay.
'america' becomes 'us' (abbreviation for the United States**).
'eastern' becomes 'e'.
'us'+'e'='USE'
(Other definitions for use that I've seen before include "Exploit unfairly" , "Bring into play" , "Put to work" , "consume; exploit" , "Expend in amusement" .)