Apparently the Shulchan Aruch rules that hand-clapping is forbidden on Shabbos, but the Rema says that some are lenient.
Rema mentions two possible grounds for leniency: 1) not wanting to inform people of their transgression if they aren’t going to stop clapping (making it an intentional transgression), 2) the fact that tosafos said that the reason behind the prohibition is no longer relevant.