First class and Underivative

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

Underivative and first class are semantically related. In some cases you can use "Underivative" instead an adjective phrase "First class".

Underivative adjective - Not derivative or imitative.

First class and underivative are semantically related. Sometimes you can use "First class" instead an adjective "Underivative".