Is there a way to completely free memory in TCL, meaning to reallocate it back to the operating system without closing the tclsh.85 process??
If you're doing this because you're about to unload the tcl DLL from your process, there's the Tcl_Finalize() API call. Otherwise, no, there's no way to do it by default. If you build Tcl with the PURIFY symbol defined (named for a commercial tool that does memory analysis) then Tcl will use the system memory allocator instead of its own high-performance thread-aware allocator; that might be able to release memory back to the OS. Or it might not; it's out of Tcl's hands entirely.
There's no script-level API for this at all. Most of the time, if a process uses a lot of memory it's either going to exit soon, going to reuse that much memory, or can rely on the OS just paging the unused memory out.