American flag over old Spanish kingdom (6)
I believe the answer is:
aragon
'old spanish kingdom' is the definition.
(Aragon was a historical kingdom)
'american flag over' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'flag' becomes 'rag' (a flag might be a rag).
'over' becomes 'on' (physically on top of).
'a'+'rag'+'on'='ARAGON'
(Other definitions for aragon that I've seen before include "Catherine of ...... was first wife of Henry VIII" , "French linesman" , "Former Kingdom in Spain" , "Old Spanish kingdom" , "part of Spain" .)