That he is an honest manis known to all. Here the underlined part is a/an -

• That he is an honest man is known to all. Here the underlined part is a - Noun clause.

• A noun clause is a group of words that band together and act like a noun. Nouns clauses are used when a single word isn't enough. They're always dependent clauses.

• একটি বাক্যের চারটি স্থানে Noun clause  বসতে পারে-
1. subject হিসাবে 
2. transitive verb এর object হিসেবে
3. Linking verb এরপরে complement হিসাবে 
4. Preposition এরপরে

• প্রদত্ত বাক্যে subject হিসাবে বসাতে এটি একটি Noun clause.