What are noun clauses
Here is the answer to your query:
A noun clause has a subject and verb. It functions as a noun in a sentence.
In the sentence, 'The Namesake is written by Jhumpa Lahiri,' The Namesake is the name of a book and thus considered to be a noun. However, if the sentence would have been 'The book I was reading last night is written by Jhumpa Lahiri' then the entire underlined part would be a noun clause which would act as a noun.
Hope it clarifies your doubt with the definition of noun clause.