Immigration Clearance by current employer

I am changing jobs T2 ICT to T2 G and resigned from my current job. My employer is supposed to complete a Clearance process, which include Immigration clearance.
They mentioned, until I tell them the details of my future visa and sponsor they are not going to provide my Immigration clearance. Wierd ...
I do not want to tell them the details. What should I do?

your sponsor's only duty to the HO is to inform them within 10 working days of the actual termination that your employment finished. This is part of their sponsor duties and if they fail to do it there may be serious consequences for them. You don't have to tell them anything about what you are going to do afterwards.