Sorry for another nooby question. How can I make "toolsclip" thing work with cubes and not just with player?

Tie the brush to a *func_clip_vphysics*, and apply a clip texture. Although it works without having the clip texture, it's always a good practice to apply the texture as it's main purpose is to indicate that there is a clip in place.

Note, however, that a func_clip_vphysics will not block the player. If you want to block everything, you can create a second player clip brush on top of your physics clip one, or just use a func_brush with nodraw or invisible.

Or just use the invisible texture, (but tie it to a func_detail (or something other), because it splits the visleaves)