Sinister group stifling publicity -- one may be criminal (7)
I believe the answer is:
culprit
'be criminal' is the definition.
(I've seen this before)
'sinister group stifling publicity one' is the wordplay.
'sinister group' becomes 'cult' (a cult might be sinister).
'stifling' is an insertion indicator.
'publicity' becomes 'pr' (abbreviation for public relations).
'one' becomes 'i' (Roman numeral).
'pr'+'i'='pri'
'cult' going around 'pri' is 'CULPRIT'.
'may' is the link.
(Other definitions for culprit that I've seen before include "The one responsible for a crime" , "baddie" , "Offender or guilty party" , "Perpetrator of a wrongdoing" , "One perpetrating crime" .)