If your client applies for ClozeSure in advance and then their sale collapses and contracts aren’t exchanged, are they entitled to a refund of their fee?

Yes, they will receive a full refund of their fee

ClozeSure will start on the day your client exchanges contracts for the sale of their property and confirm receipt of the 10% deposit paid by their buyer.  Your client's conveyancer will need to confirm both exchange of contracts and receipt of the 10% deposit. 

If the sale collapses prior to exchange of contracts, they will be entitled to a full refund of their fee.