Class, one lacking in brilliance (5)
I believe the answer is:
genus
'class' is the definition.
(thesaurus)
'one lacking in brilliance' is the wordplay.
'one' becomes 'i' (Roman numeral).
'lacking in' suggests deleting specific letters.
'brilliance' becomes 'genius' ('genius' can be a synonym of 'brilliance').
'genius' with 'i' taken away is 'GENUS'.
(Other definitions for genus that I've seen before include "Latin class?" , "Taxonomic grouping" , "Category between family and species" , "Taxonomic class between family and species" , "Taxonomic category above species" .)