It's forbidden being mean online, but nasty (13)
I believe the answer is:
unmentionable
'it's forbidden' is the definition.
(thesaurus)
'mean online but nasty' is the wordplay.
'nasty' is an anagram indicator.
'mean'+'online'+'but'='meanonlinebut'
'meanonlinebut' anagrammed gives 'UNMENTIONABLE'.
'being' acts as a link.
(Other definitions for unmentionable that I've seen before include "Forbidden as a topic of conversation" , "Not fit to be referred to" , "Taboo" .)