Back biter? (4)
'back biter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for molar that I've seen before include "part of the daily grind" , "Chewer and grinder" , "Grinding tooth in the back of a mammal's mouth" , "Type of tooth" , "Broad-crowned tooth" .)
(Other definitions for gnat that I've seen before include "Small bloodsucker" , "insect's" , "Insect like small mosquito" , "Something that bites" , "Woodland area" .)