Show excerpt to non-members on restricted tags

Right now if we restrict a tag to members only then the entire discussion is not visible to a non member.

so a non member doesn't have a clue that a discussion on something that he is interested is going on.

I want to know whether there is an way to enable non members to see the list of discussion but restrict the discussion to members only.

Not at the moment, no. You would need to write an extension to do so.