### Clue: cooler advertising works (6)
### Answer: PRISON 
 ### Explanation: Definition: cooler
 PR(abbrev. for "public relations";activities and material serving to advertise and promote) + [IS ON](is working, eg. an electric appliance).
Defn:  …, in slang.
Thatcher's son Mark was arrested, tried, fined and given a suspended jail sentence in South Africa.