It might be a variant spelling or pronunciation of 'claggy'. This is a British dialect word meaning 'sticky'. This would make sense with what is said about the genache. Alternatively, 'Cleggy' is a common British nickname for someone whose surname is 'Clegg'. Maybe someone called Clegg is being informally addressed. I don't watch Bake Off so I don't know 🙂

Not sure, but maybe he meant cloggy (that is dense and sticky and causes clogging).