Partly swollen stye in part of the eye (4)
I believe the answer is:
Here is my best explanation:
'part of the eye' is the definition.
('lens' is part of 'eye')
'partly swollen stye in' is the subsidiary indication.
'in' says the answer is hidden in the clue.
'lens' is hidden within 'partly swollen stye'.
(Other definitions for lens that I've seen before include "Bit of a looker", "Bender of beams", "Part of photographer's equipment", "French city", "Optical refractor", "Transparent optical device", "One focusing light", "Focussing glass", "Aid for viewing" and "Transparent optical devices".)