A saint going off the rails termed 'not half wicked' (9)
I believe the answer is:
satanical
'wicked' is the definition.
'a saint going off the rails termed not half' is the wordplay.
'going off the rails' indicates anagramming the letters (the order of letters is out of control).
'termed' becomes 'called'.
'not half' suggests halving the letters.
'called' cut in half is 'cal'.
'a'+'saint'='asaint'
'asaint' with letters rearranged gives 'satani'.
'satani'+'cal'='SATANICAL'
(Other definitions for satanical that I've seen before include "Devilish,evil" , "Devilish" , "Hellish" , "Extremely wicked" .)