Mark, feeling superior, accommodates duke and earl (6)
I believe the answer is:
smudge
'mark' is the definition.
(I've seen this before)
'feeling superior accommodates duke and earl' is the wordplay.
'feeling superior' becomes 'smug' (I've seen this in another clue).
'accommodates' indicates putting letters inside.
'duke' becomes 'd' (abbreviation).
'and' is a charade indicator (letters next to each other).
'earl' becomes 'e'.
'smug' going around 'd' is 'smudg'.
'smudg'+'e'='SMUDGE'
(Other definitions for smudge that I've seen before include "Smeared blot" , "Messy smear caused by rubbing" , "A smeared mark or stain" , "Blotch" , "Blur, blot" .)