Artist initially drew eleven straight lines from the centre to the circumference of a circle (5)
I believe the answer is:
radii
'lines from the centre to the circumference of a circle' is the definition.
(I've seen this before)
'artist initially drew eleven straight' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'initially' says to take the initial letters.
'eleven straight' becomes 'ii' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'drew' is 'd'.
'ra'+'d'+'ii'='RADII'
Can you help me to learn more?
(Other definitions for radii that I've seen before include "Spokes of a circle" , "Veins of insect's wings" , "Halves of diameter" , "Geometrical lines" , "Straight lines from centre of a circle" .)