I have custom module override product view. So in frontend I want to get action Save in
Custom/Controller/Adminhtml/Custom/Save. How can I do this?
You cannot use controllers from the backend on frontend and the other way around.
In order to use a controlled from the backend you need a valid backend session and you cannot get that on the frontend.
I advice you to create a method (in a helper or something) that does all the heavy lifting for saving your entity and have 2 controllers (one for backend, one for frontend) that call this common method.