Artist at ball captures second girl's luminosity (8)
I believe the answer is:
radiance
'luminosity' is the definition.
(I've seen this before)
'artist at ball captures second girl's' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'at' is a charade indicator (letters next to each other).
'ball' becomes 'dance' (ball can mean an event for dancing).
'captures' means one lot of letters goes inside another.
'second girl's' becomes 'i' (and letter of 'girl').
'dance' going around 'i' is 'diance'.
'ra'+'diance'='RADIANCE'
(Other definitions for radiance that I've seen before include "Great joy" , "Glow (of good health?)" , "Glowing quality" , "Effulgence" , "A bright glow (E, not Y)" .)