Mastermind loses one class (5)
I believe the answer is:
genus
'class' is the definition.
(I've seen this in another clue)
'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 "Division of family" , "Type" , "group of organisms" , "species grouped together" , "Taxonomic group (biol.)" .)