Yes, by using JSON serialization or by referencing an undocumented property compoundFieldName on Schema.DescribeFieldResult.
The API version of a field describe result includes this key. If non-null, the present field is a component of a compound field, whose API name is populated in that key.
This field is not (documented to be) available on Schema....
While "inlineHelpText" appears in the FieldDefintion docs it isn't in the context of a field on FieldDefinition. It appears under the CustomField Metadata. Which is the metadata you get on fields against other objects. Not against the FieldDefinition itself.
You can confirm this with the following Apex:
HttpRequest req = new HttpRequest();