America can be functional (6)
I believe the answer is:
usable
'functional' is the definition.
('usable' can be a synonym of 'functional')
'america can' is the wordplay.
'america' becomes 'us' (abbreviation for the United States).
'can' becomes 'able' (I've seen this in other clues).
'us'+'able'='USABLE'
'be' is the link.
(Other definitions for usable that I've seen before include "Like something functional" , "Not vain, presumably" , "Functional, operational" , "Practical" , "exploited" .)