Artist starts to display increasingly indistinct lines (5)
I believe the answer is:
Here is my best explanation:
'lines' is the definition.
(radius is a kind of line)
'artist starts to display increasingly indistinct' is the subsidiary indication.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'starts to' suggests taking the first letters.
The first letters of 'display increasingly indistinct' is 'dii'.
(Other definitions for radii that I've seen before include "Lines drawn from a circle's centre", "They meet in the middle of a circle", "Half diameters", "Two making up diameter", "Straight lines from the centre to the surface of a sphere", "Forearm bones: spokes", "Lines from centre to perimeter of a circle", "Arm bones", "Lines from the centre to the circumference of a circle" and "Halves of diameters".)