There's an allen-head chromed bolt that inserts from underneath the stem behind the bars. It screws into a wedge shaped piece that in turn pushes forward on the little T-shaped piece that actually compresses against the bars. The T-shaped piece, which has a stem and a square face perhaps 2 centimeters square with the curve of the bars on it, isn't held in by anything, which is why that plastic shim is put in place whenever there aren't bars inserted. Remove the shim and the T-shaped piece falls out.
The comments above about the poor clamping and the alloy expander bolt are accurate. It wasn't Cinelli's finest hour. You just might find a replacement piece at
www.bicycleclassics.com,
www.yellowjersey.org, or
www.renaissance-cycles.com; otherwise post on the vintage form and someone with a cosmetically trashed stem might have one.