Mastermind loses one category (5)
I believe the answer is:
genus
'category' is the definition.
(I know that genus is a type of form)
'mastermind loses one' is the wordplay.
'mastermind' becomes 'genius' (synonyms).
'loses' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'genius' with 'i' taken out is 'GENUS'.
(Other definitions for genus that I've seen before include "Type" , "Taxonomic group (biol.)" , "The Homo of Homo sapiens?" , "group of organisms" , "Taxonomic grouping" .)