Untruthful one (4)
I believe the answer is:
liar
'untruthful one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for liar that I've seen before include "Fibber" , "Bearer of false witness" , "One supplying pork pies" , "Story teller?" , "Worse than a thief" .)