Your command looks right but is command line stuff for after ClearOS has started. To make it permanent or to try another way, have a look at this thread. Mounts defined in fstab happen much earlier in the boot cycle.

Note that from when the bind command is used, files in olddir become invisible but are not lost. That is why Ben moved them first before rereading the fstab file with the "mount -a" command.

how can i use or exact mount command bind to move directory of flexshare to other partition

mount --bind olddir newdir?

means
mount --bind /var/flexshare/shares to new directory drive?

It can be done. Have a look at the Storage Manager app and its documentation. If you don't want to use the app, research the "mount" command using the "bind" option. It is not difficult. Alternatively you can mount your other partition directly into /var/flexshares/shares (or /var/flexshare/shares - I can't remember which). This is easier but less flexible.