Business model experimentation for sustainability is an intentional and systematic approach to identify, test and learn about value creation strategies that could be adopted by a business in response to current unsustainable trajectories. For businesses such as large clothing retailers the need to alter course is acute as pressure builds from economic, environmental and social angles. The circular economy concept provides a potential powerful lever for change. To date, however, scant research has been conducted on how circular business model experimentation is conducted. We present an in-depth action research case study of a large international clothing retailer embarking on a journey of business model experimentation for circularity: the processes, methods, roles and the organisation in light of the need to address broad sustainability challenges in the business. It was found that experimentation activities oscillated between slow and loaded, and faster and un-weighted modes. While an intentional and stepwise process was sought, in reality it was largely emergent and highly iterative. Through this iterative process, new circular business models were generated that co-exist with current non-circular ones. Confidence in experimentation as a business capability also increased through this collaborative project. This study provides insights into how to conduct lean startup type business model experimentation for circularity in a large organisation. For practitioners, the benefits of academic-industry collaboration, and the oscillating dynamics of business model experimentation are illuminated.