Untruthful ones (5)
I believe the answer is:
liars
'untruthful ones' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for liars that I've seen before include "Prevaricators" , "Deceivers" , "Untruthful people" , "Tellers of deliberate untruths" .)