What is this fossilized claw found in a Belgian forest?

Looks like the proximal end of a rib of a large mammal (cow, horse, deer). Without information about how large it is, it is not possible to get closer to an identification. It's not a claw and very likely not a fossil. It just looks old and dirty.

Here's a horse rib cage (from https://www.rodnikkel.com/content/saddle-tree-blog-from-shop-and-desk/the-rib-cage/):

Ribs in the 4-7 range seem to be a good match for shape but not as much for roundness.

That's the distal end of an antler of some deer. You can tell because the texture is smooth and glossy, all wrong for a rib, and has that flange at the end which looks very different from the uncinate processes you see in some ribs but looks a lot like the flanges you see on some deer antlers. The more rugose, "veiny" texture near the base is also typical of deer antlers. My guess is that it's either a red deer (Cervus elaphus) or roe deer (Capreolus capreolus) antler fragment.

It's not a claw, because it lacks a clear flexor tubercle, articular facet, etc.

The cross-section is also very typical of a deer antler. This is what a deer antler looks like in cross-section.