This paper examined leadership practices which supported the diffusion of an innovation in a cluster of schools in Singapore, through the lenses of complexity leadership theory (CLT) and ecological leadership. The approach is a qualitative case study, with the unit of analysis bounded by the innovation and a cluster of schools involved in the diffusion effort. The case study involved investigations mainly at four ecological levels: the ministry (macro), the cluster (exo), school/subject department (meso) and teacher (micro), involving nine observations of the cluster's community of teachers in 2019, and interviews or focused group discussions with 33 participants, including ministry officers, school leaders, key personnel and teachers. Findings and Implications: The findings illustrate the diffusion of an innovation through the interactional dynamics of administrative, adaptive, and enabling leadership, how these three CLT roles were performed by formal and informal leaders, deliberately or emergent, and across ecological levels. These leadership roles enabled learning and adaptions across and within ecologies. The study also reinforced the importance of the moral and emotional aspects of leadership in providing teachers with the motivation and support to cope with changes. The affordances, challenges, and limitations in applying CLT are elaborated.