Falsifier (4)
I believe the answer is:
liar
'falsifier' 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 "Rail at a prevaricator" , "Untruthful person (Billy in Schlesinger film)" , "One telling untruths" , "Fibber" , "Teller of untruths" .)