A poison pen is any maliciously written letter. I’ve always heard it in relation to an overly harsh, negative review. So, this is a terrible pun on that.
And more complicated variants, like anagrams, mashing words together, etc.
Additionally, I would say that that, because the answer isn't a coherent standalone word/phrase, that particular clue/answer combo would only be found in a puzzle as part of a hidden theme shared by a number of 'special clues' throughout the crossword, with some kind of rule uniting the answers (add 'ne' to the end of a common phrase, for example).