Taboo being mean online but funny (13)
I believe the answer is:
unmentionable
'taboo' is the definition.
(something taboo is not to be mentioned)
'mean online but funny' is the wordplay.
'funny' is an anagram indicator.
'mean'+'online'+'but'='meanonlinebut'
'meanonlinebut' with letters rearranged gives 'UNMENTIONABLE'.
'being' is the link.
(Other definitions for unmentionable that I've seen before include "Forbidden as a topic of conversation" , "Not fit to be referred to" , "Taboo" .)