Mastermind supporters to stay at home (6)
I believe the answer is:
brains
'mastermind' is the definition.
(I've seen this in another clue)
'supporters to stay at home' is the wordplay.
'supporters' becomes 'bras' (a bra gives support to breasts).
'to stay' is an insertion indicator (stay can mean to restrain or control).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'bras' placed around 'in' is 'BRAINS'.
(Other definitions for brains that I've seen before include "Intellectual capacity (to be picked?)" , "Minds" , "Common sense" , "Substance of the mind or intellect" , "They're all in your head" .)