Arrest nation's number one baseless conspiracy theorist (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

First letter (number one) of N [ation], then ICK [e] (David Icke, former sports journalist but now better known for supporting various dubious beliefs and conspiracy theories) without E (e = symbol for the base of natural logarithms in mathematics). Arrest, as a verb = nick (slang) = take into police custody.