The answer to your question would be that the complete adjective clause for this sentence is "I drive". In this case, the relative or adjective clause is just the words "I drive", but the word "that/which" is implied. Therefore, this is an elliptical. The technical name for a missing relative pronoun is an <em>elliptical relative pronoun. </em>
The definition of smidgen is "a small amount of something", so in this case, it means that Charlotte had a very small piece of cake at the birthday party.
Hope this helps! :)
