You've never seen this king with a tattered robe on (6)
I believe the answer is:
oberon
'you've never seen this king' is the definition.
'oberon' can be an answer for 'king' (I've seen this before). I am not sure about the remainder of the definition.
'with a tattered robe on' is the wordplay.
'with' means one lot of letters goes inside another.
'a tattered' is an anagram indicator (a tattered or broken form of the letters).
'robe' anagrammed gives 'bero'.
'bero' going within 'on' is 'OBERON'.
Can you help me to learn more?
(Other definitions for oberon that I've seen before include "Titania's king" , "Play King" , "Shakespearean role" , "Borneo (anag)" , "Shakespeare's fairy king" .)