Optical illusion partly inverted colour (5)
I believe the answer is:
lilac
'colour' is the definition.
(I've seen this before)
'optical illusion partly inverted' is the wordplay.
'partly' says the answer is hidden in the clue.
'inverted' says the letters should be written in reverse.
'calil' is hidden within 'optical illusion'.
'calil' in reverse letter order is 'LILAC'.
(Other definitions for lilac that I've seen before include "Spiritual knowledge" , "Tree with pink blossoms" , "Shrub with fragrant pink, purple or white flowers" , "Flower celebrated by Schubert" , "Fragrant,purple flower" .)