There Rema in Orach Chaim 695:2 brings that one should have mishteh vesimcha ketzas on Shushan Purim as well. The source is a pasuk in the Megilah itself. Is this therefore a chiyuv (obligation) and if so, what does it mean ketzas?
This is not a full “obligation.”
The idea is not mentioned in the Rambam and the Shulchan Aruch, and of course it is not found in the Gemara. Moreover, the Beis Yosef (end of 697) and the Darchei Moshe Ha-aroch write that this is a “custom.”
Even the Rema, who cites the Minhagim as stating that there is an “obligation” of mishteh ketzas, adds “and so is the custom” — meaning that we do not see this as a full obligation, but that this is the custom.
A source from the Megillah is mentioned (see also Biur Ha-Gra), but if we assume that this is only a custom, the derashah will not be considered a full derashah, and it is indeed not found in Chazal.
The definition of ketzas is a little more than usual. If one usually doesn’t drink during a meal, a cup of wine can be drunk, a dessert added — and so on.