In the list of such people given by Demosthenes 18.295. The actual glossing phrase here derives (as we can see from the intermediate source, Harpokration s.v.) from Theopompus FGrH 115 F119.
Kerkidas was an Arkadian, from Megalopolis, and, together with the rest of those whom Demosthenes castigates as traitors to Greece, was later defended by his pragmatic fellow-citizen Polybios (18.14).
On his more celebrated descendant and namesake, Cercidas the C3 lawgiver and poet, see OCD(4) s.v.