Mark small extract from Eugene O'Neill (10)
I believe the answer is:
apostrophe
This clue is a double definition.
'mark' is the first definition.
(punctuation mark)
'small extract from eugene o'neill' is the second definition.
(Eugene O'Neill contains an apostrophe)
(Other definitions for apostrophe that I've seen before include "Part of man's" , "address?" , "At which point discarded letters" , "Often misplaced punctuation mark" , "greengrocer abuses it!)" .)