One of two at the back of the mouth (6)
I believe the answer is:
tonsil
'one of two at the back of the mouth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tonsil that I've seen before include "One often removed" , "organ" , "Tissue at back of throat" , "One of two pieces of tissue on the pharynx" , "One of two pieces of tissue in the throat" .)