Mechanism for the chlorination of Fluorene with NCS?
According to this paper here

it has been described recently that at very low pH ($\mathrm{pH &lt;  3}$)  $\ce{Cl2}$  functions  as  an  active  chlorinating  agent

In the authors' view the system works by $\ce{NCS}$ oxidising chloride to chlorine which then provides $\ce{Cl+}$ as the chlorinating agent.