Clerk: How can I help you with?
Me: Thank you. I'm being served.

or

Me: Thank you, but the other clerk is already serving me. 聼起來自然嗎?

The second one sounds very formal but it is the correct answer.

You can also sound more casual with "Thanks, but there's already someone helping me." or "Thank you but I'm already being served/assisted/helped."

Also: The clerk would usually say "How can I help you?" Or "What can I help you with?"