Fibber (4)
I believe the answer is:
liar
'fibber' is the definition.
(fib means to lie)
This is all the clue.
(Other definitions for liar that I've seen before include "Person regularly 'economical with the truth'" , "One not telling the truth" , "One telling untruths" , "Untruthful person (Billy in Schlesinger film)" , "Teller of untruths" .)