Printing error in naughty postcard (4)
I believe the answer is:
typo
'printing error' is the definition.
(I've seen this before)
'in naughty postcard' is the wordplay.
'in' says the answer is hidden in the clue.
'TYPO' can be found hidden inside 'naughty postcard'.
(Other definitions for typo that I've seen before include "Misprint" , "Mistake in printed matter, in short" , "Literal error in printed matter, in short" , "Character flaw" , "Small keyboarding error" .)