Piece that's dark, by the sound of it? (6)
I believe the answer is:
knight
'piece' is the definition.
(chess piece)
'dark by the sound of it?' is the wordplay.
'dark' becomes 'night' (night is a kind of dark).
'by the sound of it?' shows a homophone (sound like).
'night' sounds like 'KNIGHT'.
'that's' is the link.
(Other definitions for knight that I've seen before include "Give title to; chess piece" , "Old freelance" , "One entitled to use "Sir"" , "eg Elton John" , "Holder of non-hereditary title in UK" .)