Diagnosis
That plastic stem is a weak point, and cracking is usually from overtightening or cross-threading the filter during installation. Unfortunately, the filter element is a sealed unit.
How to Fix It
You need a new Black Berkey filter element. The crack compromises the seal and the structural integrity, so it can't be reliably repaired. Carefully unscrew and remove the broken filter to avoid leaving plastic shards in the receiving nut.
If That Didn't Work
Not applicable. A cracked stem is a terminal failure for that filter. The only "fix" is replacement.
Heads Up
When installing the new filter, remember: hand-tighten only. The rubber grommet creates the seal, not extreme torque. Start the threads carefully by hand to avoid cross-threading, which is a prime cause of these cracks.