Afaik, there isn't this feature, because the :clear command doesn't accept other parameters.
Anyway, I suggest you, because it's a change request, to create as issue in the GitHub Neo4j Browser page, otherwise this one will be lost in the web ๐
When posting the feature request, could you describe the behavior that you'd prefer? Also, it'd be helpful to know what your workflow is like, to understand the context.
A few options:
modify the default behavior to clear all except for pinned frames
accept a parameter to :clear like perhaps :clear unpinned, :clear all, or :clear pinned depending on the default behavior (as @giuseppe.villani suggests)