I think the way I would approach the question is:
- Choose an example that allows you to talk extensively about the question and your experience in client service. This can be a corporate position but also waitressing or working in a shop, even those experiences will be very good examples for this question.
- Describe exactly what the question asks you, proving an exhaustive picture of what were you doing, what was your task, how did you delivered it exceptionally? How did the client feel about your delivery of the service? What was the outcome? Try to answer all of these questions whilst describing the experience and the event you have in mind.
- At the end of your answer, if you have some words left, you could explain very briefly how your client service skills will benefit you whilst working as a trainee at the firm, but this should be the focus of your answer as the question specifically asks you to focus on an example.
I hope these tips are helpful and good luck with the application!