Silences heartless criminal groups (4)
I believe the answer is:
gags
'silences' is the definition.
(gagging is a kind of silencing)
'heartless criminal groups' is the wordplay.
'heartless' suggests removing the centre.
'criminal groups' becomes 'gangs' (I've seen this before).
'gangs' with its middle taken out is 'GAGS'.
(Other definitions for gags that I've seen before include "Muffles with slangy jokes" , "Prevents from speaking the jokes" , "One-liners" , "Muzzles or jokes." , "Jokes - silences" .)