I wish to download and install the Calc program only, for use with some of my accounting clients. Is it possible to do this?
Yes and no. Under Windows if you do a Custom Install you can select Calc only. This means that in the end you will have a program group or a shortcut to Calc only. So from a user’s perspective you only installed Calc 
However since LibreOffice is a single executable, all the modules are available (even if there is no obvious link) and you won’t save any hard disc space by selecting Calc only.
I’m not sure about Linux or Mac but I believe that at least for Linux you can install Calc only.