Sorting after aggregation in Elasticsearch

You almost had it. You just need to add an order property to your a1 terms aggregations, like this:

GET myindex/_search
{
  "size":0,
  "aggs": {
    "a1": {
      "terms": { 
        "field": "FIELD1",
        "size":0,
        "order": {"a2": "desc"}      <--- add this
      },
      "aggs":{
        "a2":{
          "sum":{
            "field":"FIELD2.SUBFIELD"
          }
        }
      }
    }
  }
}