Teller of untruths (6)
'teller of untruths' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for liar that I've seen before include "Story teller?" , "Pinocchio, at times" , "Teller of fibs" , "One not telling the truth" , "Worse than a thief" .)
(Other definitions for fibber that I've seen before include "Storyteller" , "Person telling small lies, perhaps a child" , "Teller of white lies" , "One who tells porkies" , "Liar" .)