Partly swollen stye in part of the eye (4)
'part of the eye' is the definition.
('lens' is part of 'eye')
'partly swollen stye in' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LENS' is hidden in the letters of 'partly swollen stye'.
(Other definitions for lens that I've seen before include "Part of photographer's equipment", "Refracting device", "Bender of beams", "One focusing light", "French town".)