Crossword Genius

Upfront payment for dental treatment (8)

Ross

I believe the answer is:

retainer

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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!)" .)

I've seen this clue in The Hindu.
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