Rail about fibber (4)
I believe the answer is:
liar
'fibber' is the definition.
(I've seen this in another clue)
'rail about' is the wordplay.
'about' shows that the letters should be reversed in order.
'rail' backwards is 'LIAR'.
(Other definitions for liar that I've seen before include "Tale-teller" , "Perjuror" , "One supplying pork pies" , "one telling a story" , "One telling untruths" .)