{
    "requestHeader": {
       "specVersion":"1.2",
       "customerId":"123123"
     },
     "someId": "123"
}

import com.fasterxml.jackson.databind.ObjectMapper;
...
ObjectNode response = mapper.createObjectNode();
response.set("unwantedRootKey", data);

{
    "unwantedRootKey": {
          "requestHeader": {
              "specVersion":"1.2",
              "customerId":"123123"
          },
          "someId": "123"
    }
}