From Longman Dictionary of Contemporary Englishclient stateˌclient ˈstate noun [countable] PGa country that is dependent on the support and protection of a more powerful country – used in news reports
Examples from the Corpus
client state• It is seen as a client state of an overmighty United States, which drags poodle Britain in its wake.• Alternatively they risk becoming the client state of one or other adversary in a high-risk tension area. 