False witness who turns to abuse (4)
I believe the answer is:
liar
'false witness' is the definition.
(I've seen this in another clue)
'turns to abuse' is the wordplay.
'turns' is a reversal indicator.
'to abuse' becomes 'rail' (railing is a kind of abusing).
'rail' written backwards gives 'LIAR'.
'who' is the link.
(Other definitions for liar that I've seen before include "One telling untruths" , "one telling a story" , "27 [DECEIVER]" , "One not telling the truth" , "One supplying pork pies" .)