symbology - Shortening line geometry using expression dialog in QGIS?

I use the "Offset Point Symbol" tool and a geometry generator symbology to create dynamic leader lines. A sample file is available at http://hub.qgis.org/issues/15201 (Add snapping for 'Offset Point Symbol' tool).

Because I need a circle symbology without fill in some projects, I want to shorten the leader lines by the radius of the cirle symbology.

Can anyone help me with the expression? Symbology units are in meter ('Map units').

make_line (
make_point(
$x + regexp_substr("offset", '([^,]*)'),
$y - regexp_substr("offset", '([^,]*$)')
),
$geometry
)

---

Based on the answer posted by underdark I've created an expression that works properly:

-- radius = 2m

difference(
    make_line (
        make_point(
            $x + regexp_substr("offset", '([^,]*)'),
            $y - regexp_substr("offset", '([^,]*$)')

        ),
        $geometry
    ),
    buffer(make_point(
            $x + regexp_substr("offset", '([^,]*)'),
            $y - regexp_substr("offset", '([^,]*$)')
        ), 2)
  ) 

Answer

Your requirements seem to be similar to the requirements I had for a flow map a while ago. To shorten the lines at the start and at the end, I used geometric difference functions as shown here:

difference(
  difference(
    $geometry,
    buffer( start_point($geometry), 10000 )
  ),
  buffer( end_point( $geometry), 10000 )
)