Effective way to update a column to null (based on a condition) quickly in a huge 800 GB table

Hi,

I have a huge 800 GB table and need to quickly set a CLOB column of that table - which is taking up all that space to null. Now there is a criterial for this update - only based on a condition I have to set it null. Like if the length of that column is more than x bytes then set it to null. The question is that : if I do a simple UPDATE, for that CLOB column and which will affect about 600 GB, can that space be effectively reused for new inserts updates in that same