Clever person dismissing one classification (5)
I believe the answer is:
genus
'classification' is the definition.
(thesaurus)
'clever person dismissing one' is the wordplay.
'clever person' becomes 'genius'.
'dismissing' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'genius' with 'i' taken away is 'GENUS'.
(Other definitions for genus that I've seen before include "Taxonomic group (biol.)" , "Kind or sort of" , "Taxonomic class between family and species" , "Taxonomic grouping" , "Type" .)