A fastener for securing an undercover panel to an underside of a vehicle is disclosed. The fastener includes a body having a bore for receiving a threaded bolt extending from the underside of the vehicle, a flange for engaging a first side of the undercover panel, a plurality of resiliently deformable engaging fingers for allowing insertion of the bolt into the bore and to resist removal of the bolt from the bore, and a plurality of resilient engaging members for enabling the engaging members to pass in a first direction through an aperture in the undercover panel, and to resist removal of the fastener from the panel. A plurality of reinforcing members extend from the body to engage a respective engaging member, as a result of forces urging said undercover panel towards the underside of the vehicle, to resist flexing of the engaging members away from the body.