False witness who turns up to abuse (4)
I believe the answer is:
'false witness' is the definition.
(I've seen this in another clue)
'turns up to abuse' is the wordplay.
'turns up' shows that the letters should be reversed in order.
'to abuse' becomes 'rail' (railing is a kind of abusing).
'rail' in reverse letter order is 'LIAR'.
'who' is the link.
(Other definitions for liar that I've seen before include "Bearer of false witness" , "Phoney" , "Not a truth-sayer!" , "Teller of untruths" , "One not telling the truth" .)