Upfront payment for dental treatment (8)
I believe the answer is:
retainer
But I can't explain it at all! Could you help me understand?
(Other definitions for retainer that I've seen before include "Old servant - fee paid in advance" , "A money deposit or an old servant" , "Advance fee to reserve someone's services" , "Long-serving member of staff" , "Household servant (often faithful and old!)" .)