Determine if the insert is loose, if it is it will need to be re-glued.
See Vids guide on play field restoration.
I used 2-part Automotive clear coat and an industrial syringe. But this was during an entire playfield clear coat.
On Grand Prix every insert was cupped so was glad a playfield protector was available. There may be one for your machine.
Some people have cut small circles of mylar and evened out a cupped insert.
It will be very intricate and tedious to fill a cupped insert or multiple inserts with clear coat or epoxy and sand it down level without making a mess unless you plan to go through all the steps for a clear coat job.

I tried automotive clear on inserts where I wasn't clearing the whole field. I didn't want to sand flat and loose the lettering. Mine were loose so I tried one on the bench. It takes a lot more clearcoat than I thought. I ended up buying new inserts, sanding flat and making waterslide decals. Thats as far as I've gotten. I haven't attached the decals yet or tried in a game.

I 2nd the playfield protector. I put one on my Cleopatra..made a world of a difference. I haven't found one for Buccaneer yet unfortunately.