Definition of sustainability leaders
People and organisations who are clear that climate change, loss of biodiversity, reductions in non-renewable fossil fuels, depletion of rare earth metals and other significant shifts are serious and urgent challenges to which human society needs to respond.
Some leading businesses are now keen for general rules of conduct to encourage pro-environmental behaviours, and seek to influence policy in field-shaping ways. Often they are involved in cross-sector initiatives to integrate environmental, social and economic benefits.
Courage and risk-taking are key components of sustainability leadership. But so is taking care of oneself. Burnt-out 'change agents' to society are not sustainable models. People work at developing the crafts of doing this work well, including skills of communication. 
Some examples of sustainability leaders include: people pioneering responsible policies and practices in their organisations and building appropriate coalitions, often across sectors.
One specific example of a sustainability leader is Marks and Spencer with its 'Plan A' programme (see article link below).
Article: Marks and Spencer's green blueprint