Mark on a black beetle (6)
I believe the answer is:
scarab
'beetle' is the definition.
(scarab is a kind of beetle)
'mark on a black' is the wordplay.
'mark' becomes 'scar' ('scar' can be a synonym of 'mark').
'on' is a charade indicator (letters next to each other).
'black' becomes 'b' (as in pencil classifications).
'scar'+'a'+'b'='SCARAB'
(Other definitions for scarab that I've seen before include "divine thing once" , "old lucky charm" , "Sacred Eygptian beetle" , "Sacred dung-beetle" , "Beetle revered in ancient Egypt" .)