loks like a broken tab that holds the lens into the body of the lamp.
The tail light is a funny critter. There is no way it just slots in and out of the grabrail section without some manipulation.
I found out with trial/error and then plenty of helpful advice here on the forum.
I was tryiny to fit an LED taillight and could not for the life of me figure how to get it in to the grabrail(square peg - round hole dilemma)
I almost took the lens out of the bdy to do it and the end result would probably be exactly what you have now. the lens and body are a reasonably thick but brittle plastic designed to be fitted to gether initially and not taken apart.
TRhe method to fit or remove a tailight assembly requires turning the grabrail uside down putting pressure on both rails to force the tail light aperture apart just enough for the light to slide in/out easily. Best with two people but can be done by one.
Looking at what you have, you could pop the lens and refelctor parts of the tail light, re-glue as needed and re fit.
as well as some others I have used some bits of rubber to minimise the amount of movement of the lamp assembly in the tail grabrail
good
luck with it
