Slander male in a variety of lunacy (7)
I believe the answer is:
calumny
'slander' is the definition.
('calumny' can be a synonym of 'slander')
'male in a variety of lunacy' is the wordplay.
'male' becomes 'm' (common abbreviation).
'in' means one lot of letters goes inside another.
'a variety of' indicates an anagram.
'lunacy' is an anagram of 'caluny'.
'm' inserted inside 'caluny' is 'CALUMNY'.
(Other definitions for calumny that I've seen before include "Untrue and malicious statement about a person" , "Slander, defamation" , "Abuse" , "A slanderous statement" , "False accusation or statement" .)