Clue: Thinker going round in flat naked (4)
Answer: KANT
Explanation: Definition: Thinker
Hidden backwards in [*going round in*] "fla**T NAK**ed" 
 
 
*Immanuel Kant (1724-1804) was a German Enlightenment philosopher, who according to Monty Python, was "a real pissant who was very rarely stable"