Girl, behold, one in America (6)
I believe the answer is:
louisa
'girl' is the definition.
'behold one in america' is the wordplay.
'behold' becomes 'lo'.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'america' becomes 'usa' (synonyms).
'i' put into 'usa' is 'uisa'.
'lo'+'uisa'='LOUISA'
(Other definitions for louisa that I've seen before include "-- May Alcott, author" , "girl" .)