jolt transform spec ?

[l vic]

I have json message that contains another json message in textual form:
{
"one": "one".
"two":2,
"MSG": "{\"src\":\"my source\",\"priority\":\"0\"}"
}

What would be transform spec to get "contained" message in json ?
{
"src": "my source",
"priority": "0"
}

I've tried the following spec:
[
  {
    "operation": "shift",
    "spec": {
      "MSG": {
        "*": "&"
      }
    }
  }
]
but ended up with just text message:
{ "MSG": "{\"src\":\"my source\",\"priority\":\"0\"}"}
How should i change it?
Thank you