Violent tough guys kept in check (8)
I believe the answer is:
vehement
'violent' is the definition.
('vehement' can be a synonym of 'violent')
'tough guys kept in check' is the wordplay.
'tough guys' becomes 'hemen' (I've seen this in other clues).
'kept in' means one lot of letters goes inside another.
'check' becomes 'vet' (I've seen this before).
'hemen' placed within 'vet' is 'VEHEMENT'.
(Other definitions for vehement that I've seen before include "Even them being intense, showing strong feeling" , "Forcible" , "Forcefully emphatic" , "Forceful, emphatic" , "Forceful, ardent" .)