Artist starts to display increasingly indistinct lines (5)
I believe the answer is:
radii
'lines' is the definition.
(radius is a kind of line)
'artist starts to display increasingly indistinct' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'starts to' indicates taking the first letters.
The first letters of 'display increasingly indistinct' is 'dii'.
'ra'+'dii'='RADII'
(Other definitions for radii that I've seen before include "Lines between the center and circumference of a circle" , "Half diameters" , "Halves of diameter" , "Lines from a circle's centre to the circumference" , "Forearm bones: spokes" .)