The claim has never been independently made outside of True Pundit, and it cited anonymous State Department sources.
Clinton claims not remembering saying that.
I don't recall any joke. It would have been a joke if it had been said, but I don't recall that.
It very unlikely that killing Assange with a drone was a serious suggestion, as he was residing in non-hostile countries and embassies that that were unwilling to extradite; they would probably be very upset by a drone kill. Other more conventional methods of killing or capturing seem more plausible.
It could have been said as a rhetorical question for taking decisive (possible lethal) action. There's really no verifiable way to know. "Extraordinary claims require extraordinary evidence."