Create a new column by group

    Table.AddColumn(
        Source, "Custom", 
        each Text.Trim(
            if Text.EndsWith([country], "- AC") 
            then Text.Range([country], 0, Text.Length([country]) - 4) 
            else [country]
        )
    )

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlTSUcpKLEjMUwAyHJ2AhKGBgYFSrA6ylIKugqMzTN4USdo3tSIzOR8m7ewCJMyMEdKhwY5wU02g2pKQLQTrgFuIkEI2ECEHNA7FJSgy2LVAHAh3BLLbkUyDONwAVRLZJhOwXCwA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [group = _t, country = _t, #"type" = _t, value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"group", type text}, {"country", type text}, {"type", type text}, {"value", Int64.Type}}),
    #"Extracted country" = Table.TransformColumns(#"Changed Type", {"country", each Text.Trim(Text.BeforeDelimiter(_,"-"))})
in
    #"Extracted country"