You prompt the LLM to point out that the clue will be added as is to the conversation, but for the LLM to include a marker instead of the actual text to ensure that actual critical details are included unchanged.

EDIT: Also, having the LLM botch a clue occasionally could be a feature. E.g. a bumbling character that you might need to "interrogate" a bit before you actually get the clue in a way that makes sense, and can't be sure it's entirely correct. That could make some characters more realistic.