How can I clean/delete a Jenkins workspace after merge?
I am assuming you are using Jenkinsfile. You can do for example:

pipeline {
    agent any

    stages {
        stage('Build') {
            steps {
                echo 'Building..'
            }
        }
        stage('Test') {
            steps {
                echo 'Testing..'
            }
        }
        stage('Deploy') {
             when { branch 'master' } /* when in master branch */
             steps {
                echo 'Deploying....'
                deleteDir() /* clean up our workspace */
            }
        }
    }
}