Punctuation mark perhaps too irregular (10)
I believe the answer is:
apostrophe
'punctuation mark' is the definition.
(apostrophe is a kind of punctuation mark)
'perhaps too irregular' is the wordplay.
'irregular' is an anagram indicator.
'perhaps'+'too'='perhapstoo'
'perhapstoo' with letters rearranged gives 'APOSTROPHE'.
(Other definitions for apostrophe that I've seen before include "Mark in exam's" , "in Charley's Aunt ?" , "rhetorical digression" , "Where character may be absent" , "Passage addressed to character" .)