Insult, informally (3)
'insult informally' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dis that I've seen before include "Name for Pluto or hell" , "Speak to (someone)without respect" , "Treat without respect - god of the underworld" , "Show contempt" , "The god Pluto" .)
(Other definitions for diss that I've seen before include "On the street, treat with contempt" , "Eastern town" , "East Anglian town" , "Norfolk town" , "Be impolite" .)