Black Prince, say, is reported as something black (6)
I believe the answer is:
Here is my best explanation:
'black prince say' is the definition.
'reported as something black' is the subsidiary indication.
'reported as' shows a homophone (sound like).
'something black' becomes 'night'.
'night' is a homophone of 'knight'.
'is' is the link.
(Other definitions for knight that I've seen before include "rider looking for adventure?", "Chesspiece which moves in the shape of an L", "In Britain, a person honoured by the sovereign for personal merit", "Holder of non-hereditary title in UK", "nobleman", "Ladies' champion", "horse in game!", "The horse's head in chess", "Chevalier" and "board member".)