Expression used in 'Mastermind' (4)
I believe the answer is:
term
'expression' is the definition.
(thesaurus)
'in mastermind' is the wordplay.
'in' says the answer is hidden in the clue.
'TERM' is hidden in the letters of 'mastermind'.
'used' acts as a link.
(Other definitions for term that I've seen before include "expected time of delivery" , "Period of school life" , "Hilary for one" , "Word or expression for some particular thing" , "Scholastic period" .)