This is what works for me:
RendererManager rendererManager = ComponentAccessor.getComponent(RendererManager.class);
JiraRendererPlugin renderer = rendererManager.getRendererForType("atlassian-wiki-renderer");
String output = renderer.render(issue.getDescription(), issue.getIssueRenderContext());
Source: How to convert JIRA wiki markup to HTML programmly using Atlassian native API?