Amazed to be united under America's leader (4)
I believe the answer is:
awed
'amazed' is the definition.
(thesaurus)
'united under america's leader' is the wordplay.
'united' becomes 'wed' (I've seen this before**).
'under' says to put letters next to each other.
'leader' suggests taking the first letters.
The first letter of 'americas' is 'a'.
'wed' after 'a' is 'AWED'.
'to be' acts as a link.
(Other definitions for awed that I've seen before include "Filled with reverential wonder" , "Full of wonder" , "Inspired with wonder" , "Seriously impressed" , "Struck with fear/wonder" .)