Publicising first-class cartel (6)
I believe the answer is:
airing
'publicising' is the definition.
('air' can be a synonym of 'publicise')
'first-class cartel' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'cartel' becomes 'ring' (both can mean a criminal group).
'ai'+'ring'='AIRING'
(Other definitions for airing that I've seen before include "Exposure" , "Publicity" , "Broadcast" , "Ventilation" , "Showing" .)