Clue: Idiot favouring rubbish principle of The Avengers? (3,3,3)
Answer: TIT FOR TAT
Explanation: Definition: principle of The Avengers
 TIT(an idiot/an ineffectual person) + FOR(favouring/seconding) TAT(rubbish/cheap and bad articles like clothing or jewellery).
Defn: …, or "one bad turn deserves another".
Landmark 6: