Mischief and evil men therefore conceal (9)
I believe the answer is:
devilment
'mischief' is the definition.
('devilment' can be a synonym of 'mischief')
'and evil men therefore conceal' is the wordplay.
'conceal' indicates a hidden word.
'DEVILMENT' is hidden within 'and evil men therefore'.
(Other definitions for devilment that I've seen before include "Frolicsome mischief" , "Reckless mischief" .)