Not a direct form of slander (10)
I believe the answer is:
detraction
'slander' is the definition.
(I've seen this before)
'not a direct form' is the wordplay.
'form' indicates anagramming the letters (another form of the letters).
'not'+'a'+'direct'='notadirect'
'notadirect' is an anagram of 'DETRACTION'.
'of' acts as a link.
(Other definitions for detraction that I've seen before include "Disparagement" , "Slander" .)