I would personally break this down into a simple STAR response: Situation, Task, Action, Response/ Result. I would then link that back to why it's relevant to a career in commercial law, specifically at that firm. E.g.:
"In order to maintain an income stream while at University, I launched a successful online business. I researched my market to understand client needs, spearheaded my efforts in specific areas to ensure that my business was relevant to potential clients and subsequently launched. The business was extremely successful and highly profitable within three months of launch. [This is the STAR component]. Unfortunately, there was a regulatory change which meant I had to cease doing business. I feel that having a first-hand understanding of how law impacts on businesses would make me an ideal candidate for X firm because [list parallels between your skills/ experiences/ beliefs and the firm's aims etc]".
