Tell me frankly why you did this. the underlined part is a/an-

Tell me frankly why you did this. Underlined অংশটি 'tell'-এর object হিসেবে কাজ করছে, তাই এটি Noun clause.

Noun clause একটি subordinate clause যা noun-এর কাজ করে (subject, object, complement, apposition)।
Examples: That he has much money is known to all. (subject) I know that he has done it. (object) This is what I said. (complement) The fact that he is a thief is clear to all. (apposition)

অন্যান্য:

• Adjective clause: noun/pronoun-এর পরে বসে তাকে modify করে।
• Adverbial clause: verb, adjective বা অন্য clause-কে modify করে; time, place, cause, purpose বোঝায়।
• Adverbial phrase: phrase যা adverb-এর মতো কাজ করে এবং when/where/why/how প্রশ্নের উত্তর দেয়।