Is it possible to turn on composite function but without panel transparency (I need it for screenlets).
OK I've found it:
"Please, note that transparency only works with an X server that supports the Composite extension, like XOrg >= 6.8.0 and the composite extension has been enabled in the configuration file and the window manager supports it (e.g. xfwm4 has to be compiled with --enable-compositor).
By default the panel will be transparent unless the autohide option has been set. When the mouse moves over the panel, the transparency will be temporarily removed.
You can create the file <basedir>/xfce4/transparency if you want to change the transparency value for the panel (or the iconbox). Create the file and add a line with the format panel=<value>, where value is a number between 0 and 100. Setting it to 0 effectively turns transparency off.
For example, a transparency file may look like this:panel=25
Now I don't know what kind of file should it be? I've created txt file but it doesn't work.