Essentially alters fake ivories, smuggling in article (9)

'essentially alters' is the definition.
(changes the nature of something)

'fake ivories smuggling in article' is the wordplay.
'fake ivories' becomes 'dentures' (ivories can mean someone's teeth).
'smuggling in' means one lot of letters goes inside another.
'article' becomes 'a' (the indefinite article in English).
'dentures' going around 'a' is 'DENATURES'.