Student bile turned into slander (5)
I believe the answer is:
libel
'slander' is the definition.
(I know that libel is a kind of slander)
'student bile turned' is the wordplay.
'student' becomes 'L' (as in L-plates for learner drivers).
'turned' indicates anagramming the letters.
'bile' is an anagram of 'ibel'.
'l'+'ibel'='LIBEL'
'into' is the link.
(Other definitions for libel that I've seen before include "Write ill of" , "Written defamation" , "Actionable words" , "smear on paper" , "Report bad things" .)