A mastermind omits one kind (5)
I believe the answer is:
genus
'kind' is the definition.
(a genus is a kind or type of something)
'a mastermind omits one' is the wordplay.
'a mastermind' becomes 'genius' ('genius' can be a synonym of 'mastermind').
'omits' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'genius' with 'i' removed is 'GENUS'.
(Other definitions for genus that I've seen before include "Taxonomic group (biol.)" , "Kind or sort of" , "Type" , "group of organisms" , "Category between family and species" .)