Girl chess player, faultless (4-5)
I believe the answer is:
'faultless' is the definition.
(I've seen this before)
'girl chess player' is the wordplay.
'girl' becomes 'Lily' (girl's name).
'chess player' becomes 'white' (one of the two players in a game of chess).
(Other definitions for lily-white that I've seen before include "Without blame" , "Pure - irreproachable" , "Without fault" , "Innocent, immaculate" , "Totally innocent" .)