He was taunting her. Clara knew this yet she still fell for it. Mostly because she knew that if the idea was in fact planted in her dad's head, jokingly or not? He'd seriously consider it and then she might actually crawl into a hole in the ground because that was really the only way to respond if Duncan showed up at Creekside as caretaker or teacher.
"Well he can find his inspiration elsewhere, Troy Handsome!"
When the full name came out, well.. usually that was a bad thing. Nose scrunched up in annoyance (She wasn't actually angry. Annoyed and vaguely panicking her cousin might actually do it, yes. Angry, no) before grabbing a couch pillow and hitting him with it. Usually John was on the receiving end of the pillow attacks but Clara felt this was a good enough reason to do so.
"You'll get more than a pillow bollocking if you do."