The Avengers S02 E23 Conspiracy of Silence

"Mrs Peel, We're Needed!"

The Mafia aren't about to let Steed get in the way of their lucrative drug-smuggling operation between Britain and North America, and arrange for him to be assassinated. Steed survives, and tracks his intended killer to a circus, where some of the clowns prove to be anything but figures of fun.