Rail about fibber (4)
I believe the answer is:
Here is my best explanation:
'fibber' is the definition.
(I've seen this before)
'rail about' is the subsidiary indication.
'about' shows that the letters should be reversed in order.
'rail' written backwards gives 'liar'.
(Other definitions for liar that I've seen before include "Bearer of false witness", "Dishonest person", "Person regularly 'economical with the truth'", "Prevaricator", "Teller of untruths", "Fibber", "Untruthful person (Billy in Schlesinger film)", "Tale-teller", "The truth is not in him or her" and "One not telling the truth".)