Person who paints diameter and two lines in circle (5)
I believe the answer is:
radii
'lines in circle' is the definition.
(plural of radius)
'person who paints diameter and two' is the wordplay.
'person who paints' becomes 'ra' (abbreviation for a Royal Academy of Arts member).
'diameter' becomes 'd' (mathematics).
'and' is a charade indicator (letters next to each other).
'two' becomes 'ii' (roman numeral).
'ra'+'d'+'ii'='RADII'
(Other definitions for radii that I've seen before include "Halves of diameters" , "Lines drawn from a circle's centre" , "Half-diameters" , "Two making up diameter" , "Forearm bones: spokes" .)