Part of eye (4)
I believe the answer is:
iris
'part of eye' is the definition.
('iris' is part of 'eye')
This is all the clue.
(Other definitions for iris that I've seen before include "Fleur-de-lis" , "Flower - rainbow" , "Woman" , "is 21[WOMAN], perhaps" , "Plant with sword-shaped leaves" .)