Scottish king seen as dull mug (6)
I believe the answer is:
duncan
'scottish king' is the definition.
(I've seen this before)
'dull mug' is the wordplay.
'dull' becomes 'dun'.
'mug' becomes 'can'.
'dun'+'can'='DUNCAN'
'seen as' is the link.
(Other definitions for duncan that I've seen before include "Man's name" , "innovative dancer" , "Scottish king" , "Isadora ..., pioneer of modern dance" , "King murdered by ambitious Macbeth" .)