Northern girl and boy from a state in America (7)
I believe the answer is:
nevadan
'from a state in america' is the definition.
(from Nevada)
'northern girl and boy' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'girl' becomes 'eva' (girl's name).
'and' means one lot of letters go next to another.
'boy' becomes 'dan' ().
'n'+'eva'+'dan'='NEVADAN'
(Other definitions for nevadan that I've seen before include "One coming from Las Vegas, perhaps" , "Statesman" .)