Back tooth (5)
I believe the answer is:
molar
'back tooth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for molar that I've seen before include "Grinding tooth in the back of a mammal's mouth" , "Moral (anag.)" , "that could be capped" , "Chewer and grinder" , "Dealing with masses" .)