To import a leiningen project in the Eclipse IDE add the eclipse leiningen plugin in the project.clj as dev-dependency.
When you now invoke the command "lein help" there is a new task called eclipse. Invoke the leiningen task eclipse via "lein eclipse". The eclipse leiningen tasks creates the eclipse projects files ".project" and ".classpath".
Now you can import the project in the eclipse IDE, therefore you can use the import eclipse wizard. Therefore select "File->Import->Existing Project into Workspace" then click the next button and select the leiningen project as root directory now you can import the project via click on the finish button.
When you have installed the eclipse plugin counterclockwise you can know develop with syntax highlighting, run the project and debug the project from the eclipse IDE.