Four included in slander that's offensive (7)
I believe the answer is:
abusive
'offensive' is the definition.
(similar in meaning)
'four included in slander' is the wordplay.
'four' becomes 'IV' (Roman numerals for four).
'included in' is an insertion indicator.
'slander' becomes 'abuse' (I can't justify this - if you can you should give a lot more credence to this answer).
'iv' inserted within 'abuse' is 'ABUSIVE'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for abusive that I've seen before include "Swearing, insulting" , "Extremely insulting" , "Defamatory" , "Vituperative" , "Calumniating" .)