Decayed part of tooth (6)
I believe the answer is:
cavity
'decayed part of tooth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cavity that I've seen before include "A hollow within a solid, say tooth" , "Hollow place" , "Hole, crater" , "Hollow space" , "A hole could mean work for the dentist" .)