Exploited by America, England beginning to doubt (4)
I believe the answer is:
used
'exploited' is the definition.
('used' can be a synonym of 'exploited')
'america england beginning to doubt' is the wordplay.
'america' becomes 'us' (abbreviation for the United States).
'england' becomes 'e'.
'beginning to' suggests taking the first letters.
The first letter of 'doubt' is 'd'.
'us'+'e'+'d'='USED'
'by' is the link.
(Other definitions for used that I've seen before include "Availed oneself of" , "exhausted" , "Second-hand - not wasted" , "Of clothes or cars, second-hand" , "like majority of cars?" .)