Rail about dishonest person (4)
I believe the answer is:
liar
'dishonest person' is the definition.
(I've seen this before)
'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 "Untruthful person (Billy in Schlesinger film)" , "one trying to sell pork pies" , "Worse than a thief" , "Storyteller" , "Perjuror" .)