Mastermind loses one type (5)
I believe the answer is:
genus
'type' is the definition.
(thesaurus)
'mastermind loses one' is the wordplay.
'mastermind' becomes 'genius' ('genius' can be a synonym of 'mastermind').
'loses' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'genius' with 'i' taken out is 'GENUS'.
(Other definitions for genus that I've seen before include "Type" , "Taxonomic category above species" , "Taxonomic group (biol.)" , "Part of the family" , "Class of animal" .)