Pair of kings? You could do better (3-3)
I believe the answer is:
tut-tut
'you could do better' is the definition.
'pair of kings?' is the wordplay.
'pair' shows that letters should be duplicated.
'of kings?' becomes 'tut' (I can't justify this - if you can you should give a lot more credence to this answer).
'tut' doubled is 'TUT-TUT'.
Can you help me to learn more?
(Other definitions for tut-tut that I've seen before include "Repetitive exclamation of impatience" , "For shame!" , "Expression of disapproval - a repetitive palindrome" , "Word of disapproval" , "Mild exclamation of disapproval" .)