Manxman, for example, doesn't start defamation (7)
I believe the answer is:
slander
'defamation' is the definition.
(I know that defamation can be written as slander)
I don't understand the rest of the clue.
(Other definitions for slander that I've seen before include "Sling mud at" , "False/malicious speech" , "muckraking!" , "Malicious speech" , "Calumny" .)