Perhaps too bad a mark (10)
I believe the answer is:
apostrophe
'a mark' is the definition.
(punctuation mark)
'perhaps too bad' is the wordplay.
'bad' indicates anagramming the letters.
'perhaps'+'too'='perhapstoo'
'perhapstoo' anagrammed gives 'APOSTROPHE'.
(Other definitions for apostrophe that I've seen before include "Passage addressed to character" , "detail of punctuation" , "Where character may be absent" , "Letter omission indicator" , "address?" .)