In France, he's swallowed by lake of purplish hue (5)
I believe the answer is:
Here is my best explanation:
'hue' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
(Other definitions for lilac that I've seen before include "Fragrant shrub or tree", "Lavender colour", "flowering shrub", "Plant", "Shrub with fragment purple or white flowers", "Pale violet hue", "garden ornamental", "Light purple colour", "Flower celebrated by Schubert" and "Shrub or small tree with fragrant blossom".)