Enemy beginning to treat America as undeveloped entity (6)
I believe the answer is:
foetus
'undeveloped entity' is the definition.
(developing offspring)
'enemy beginning to treat america' is the wordplay.
'enemy' becomes 'foe' (synonyms).
'beginning to' suggests taking the first letters.
'america' becomes 'US' (United States).
The first letter of 'treat' is 't'.
'foe'+'t'+'us'='FOETUS'
'as' is the link.
(Other definitions for foetus that I've seen before include "issue" , "Embryo of a mammal" , "Embryo in later stages" , "Developed embryo in the womb" , "Human embryo more than six weeks after conception" .)