Mark a British beetle (6)
I believe the answer is:
scarab
'beetle' is the definition.
(I know that scarab is a type of dung beetle)
'mark a british' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'scar'+'a'+'b'='SCARAB'
(Other definitions for scarab that I've seen before include "Large dung beetle" , "crawler" , "divine creature" , "Sacred Egyptian beetle" , "Egyptian stone" .)