Perjurer who turns to abuse (4)
I believe the answer is:
liar
'perjurer' is the definition.
(thesaurus)
'turns to abuse' is the wordplay.
'turns' shows that the letters should be reversed in order.
'to abuse' becomes 'rail' (railing is a kind of abusing).
'rail' written backwards gives 'LIAR'.
'who' acts as a link.
(Other definitions for liar that I've seen before include "Fabricator" , "One telling untruths" , "one telling a story" , "Perjuror" , "Person telling stories" .)