Well, I went with the tried and true method: Throw an enchantment on the target when nobody's looking, then distract the rest of the group with shiny loot.
So during the session, I pass a note to the cleric's player, asking her to make five DC 21 Will saves when their character is asleep in the dragon's castle. The player's character, being a Wisdom based spellcasting class, has an absolutely massive bonus to their Will saving throws too, so they only have about a 20% chance of failure per save. Unluckily for them, one of those five rolls fails. Cue second secret note!
The pretty young cleric gets tagged with a Suggestion spell: For the next seven hours, you think it would be a great idea to stay and study longer in the dragon's library. For background, the dragon's been watching her from behind one of the paintings of himself in her room that night, one with a secret room behind it and eyes that open out to peep into her room as she's sleeping. I described the room and the paintings to the players, but apparently nobody was paranoid enough to check if they were rigged to spy on them.
Then breakfast rolls around, and the dragon offers everyone gifts before they depart. I basically throw a bunch of random low-level magical crap at them, like wands and wondrous items, but the cleric gets an offer from the dragon to enjoy an extended stay. She accepts it in front of the group as witnesses, and there's nobody who checks her for magical tampering, too busy enjoying their own loot. The dragon smiles a toothy grin as the rest of the adventurers leave his castle, seals up the entrances, and the scene closes on him standing over the cleric as she reads in his library, gently stroking her hair as he monologues with cringe-worthy creepiness.
Mission success!