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 from a circle's centre to the circumference", "Spokes of a circle", "Halves of diameters", "Lines from centre to perimeter of a circle", "Bones of the human forearm", "Half-diameters", "Straight lines from the centre to the surface of a sphere", "Lines drawn from a circle's centre", "Centre-circumference lines" and "spoke several times".)