One going in provides treatment for tooth decay (6)
I believe the answer is:
caries
'for tooth decay' is the definition.
(dental caries)
'one going in provides treatment' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'treatment' could be 'care' (I've seen this in another clue) and 'care' is found within the remaining letters.
A single letter 's' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for caries that I've seen before include "Decayed area in a tooth" , "Dental cavities" , "Dental decay" , "I scare easily with tooth decay" , "Decay of bone or teeth" .)