Word repeated before "Pants on fire!" (4)
I believe the answer is:
liar
'word repeated before pants on fire' 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" , "Fabricator" , "Untruthful person (Billy in Schlesinger film)" , "One not telling the truth" , "One telling untruths" .)