The documentation on the API Viewer specifies that:

clean(boolean clean = true)
Clean up the workspace before every checkout by deleting all untracked files and directories, including those which are specified in .gitignore. Defaults to false.

https://jenkinsci.github.io/job-dsl-plugin/#method/javaposse.jobdsl.dsl.helpers.scm.GitContext.clean

The result of using clean() is that in the Job configuration, "Clean after checkout" is selected, instead of "before"

Current outcome:
clean() --> <clean>true</clean>

Desired outcome (after manually adding the Clean Before option)
clean() -->

<extensions>
  <hudson.plugins.git.extensions.impl.CleanBeforeCheckout/>
</extensions>

If I however try to add the Clean after checkout manually, the generated xml is the following:

    <extensions>
      <hudson.plugins.git.extensions.impl.CleanCheckout/>
    </extensions>