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 "Camera part", "Focussing glass", "One focusing light", "Part of the inner eye, or camera", "Optical glass", "Piece of glass in optical instrument", "City of northern France: periscope part", "Optical element", "Focusing glass" and "Piece of glass, usually curved".)