That's the crankcase breather pod, there is a mesh filter inside which often gets blocked. Access to remove it isn't the best, and it's quite a tight fit in the block, especially a new one. You'll need to prise it away from the block without damaging the mating surface. You can clean the mesh, some people remove it altogether, some re-fit the old pod and use gasket sealant, some fit a new pod without sealant.