Go on, smile — that's rude new word (9)
I believe the answer is:
neologism
'new word' is the definition.
'go on smile that's rude' is the wordplay.
'that's rude' is an anagram indicator.
'go'+'on'+'smile'='goonsmile'
'goonsmile' with letters rearranged gives 'NEOLOGISM'.
(Other definitions for neologism that I've seen before include "Brexit maybe" , "Newly coined word" , "new term" , "Newly-coined word or expression" , "coinage" .)