Novel not with Smiley is serious stuff (9)
I believe the answer is:
solemnity
'serious stuff' is the definition.
(solemnity can mean seriousness)
'novel not with smiley' is the wordplay.
'novel' indicates an anagram.
'with' means one lot of letters go next to another.
'not' put after 'smiley' is 'smileynot'.
'smileynot' anagrammed gives 'SOLEMNITY'.
'is' is the link.
(Other definitions for solemnity that I've seen before include "Rite" , "Portentousness" , "seriousness" .)