I have cloned in multiple copy an object. I would like to modify the cloned object grouping it with another object or combining it with a another path but when I try to do it the clones don't get updated. Where do I go wrong?
 

Group will not work because the group must be cloned first.

Combine will work in the parent to the clones is the top object of the combining paths.

I'm not able to make it work.

1. Create a path.
2. Cloned.
3. Create another path. 
4. Combined the first path and the second path (using Ctrl K)
5. The clone does not get updated

Any mistakes?

Ensure the original path is brought in front of the other. i.e Step 3.5... bring up.

Try this:

Draw a path
Create a group (with one object)
Make a clone of this group
Open the group [double click]
Draw another path

The Manual says:

Inkscape has the ability to “relink” clones to a another object. To do this, copy the new “original” to the clipboard. Select all the clones to be relinked and then use the Edit → Clone → Relink to Copied command. The clones will probably shift in place. In practice, this just changes the xlink:href attribute of the clones to point to the new “original”.

First of all, sorry for my late answer!

@TylerDurden thank you for the video, it was extremely clear. And you were right, I had to "Raise selection to the top". After doing that it works very well! Great!

I tried also the "relink" option that works with group. It is a great feature but it's not practical for my need because I have many clones and it would be very difficult/time consuming select all them.

Thank you also to @Paddy_CAD. I made some test with grouping and it has not easy but at the end I have been able to add objects to a cloned group and made the clones get updated!!!

Thanks a lot for all your fantastic help!!!