The correct spelling for the phrase "be client" is actually "be a client". In IPA phonetic transcription, it would be [bi ə ˈklaɪənt]. The "a" sound is pronounced as the schwa sound /ə/. "Client" is spelled with a "c", not a "k", and it is pronounced as /ˈklaɪənt/. The phrase "be a client" means to become or act as a customer or patron of a business or service. So if you want to do business with someone, you should aim to be a client.
The term "be client" is not a commonly used phrase in the English language. It is possible that you are referring to the verb "to be" combined with the noun "client". In this case, the etymology of the word "client" originates from the Latin word "cliens", which referred to a dependent or a follower. Over time, its meaning evolved to denote someone who seeks professional services or advice, such as legal or business services. The verb "to be" has its root in Old English.