Extract of dirty poem, one showing errant character (4)
I believe the answer is:
Here is my best explanation:
'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 "Printing mistake", "Spelling mistake", "Thanks; good health!", "Literal - printing mistake", "Printer's mistake", "Small printing error", "Keyboard slip-up", "Printer's error", "Literal error in printed matter, in short" and "Mistake in printed matter, in short".)