I struggled with LILY too. May I ask whether you got the hint from "white"?
I checked "white" in Chambers, dictionary and thesaurus, but there was no mention of "lily", yet on looking up "lily" there was "white".
It seems to be back-to-front to me.
I've just checked my print copy of Chambers Crossword Dictionary, after blowing the dust off, and "lily" is included in a list of whites.