Misprint in dirty poem (4)
I believe the answer is:
typo
'misprint' is the definition.
(I know that misprint can be written as typo)
'in dirty poem' is the wordplay.
'in' says the answer is hidden in the clue.
'TYPO' is hidden in the letters of 'dirty poem'.
(Other definitions for typo that I've seen before include "Spelling mistake" , "literal, perhaps" , "Keyboard slip-up" , "Literal - printing mistake" , "Small printing error" .)