Extract of dirty poem, one showing errant character (4)
I believe the answer is:
typo
'one showing errant character' is the definition.
(a typo includes an incorrect letter or character)
'extract of dirty poem' is the wordplay.
'extract of' indicates the answer is hidden within the clue (I've seen this in other clues).
'TYPO' is hidden within 'dirty poem'.
(Other definitions for typo that I've seen before include "What's not right" , "Spelling mistake" , "Printer's error" , "Printer's mistake" , "Literal - printing mistake" .)