It's a big thing in dentistry (5)
I believe the answer is:
molar
'dentistry' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for molar that I've seen before include "that could be capped" , "Grinding tooth in the back of a mammal's mouth" , "Moral (anag.)" , "of ivory, perhaps" , "Type of tooth" .)