Indicator repeater lamp.
This is the all-plastic clip-in type, as on later models, (not the earlier metal-bodied type with two mounting studs).
These have not been available from the usual Citroën parts specialists for many years but we have one left.
The last online price we could find elsewhere was over £60.
Original Citroen part numbers: 5503138 & 95560417.
Original Valeo part number: 083657.