layers = iface.legendInterface().layers()
layer = None
for l in layers:
    if str(l.name())=='a_polygon':
        layer = l
if layer:
   crs = str(layer.crs().authid()) 
   vl = QgsVectorLayer("Polygon?crs="+crs, 'new_polygon', "memory")
   pr = vl.dataProvider()
   origFields = layer.pendingFields()
   pr.addAttributes( origFields )