What can magnify part of pollen sac (4)
I believe the answer is:
lens
'what can magnify' is the definition.
'part of pollen sac' is the wordplay.
'part of' says the answer is hidden in the clue.
'LENS' is hidden within 'pollen sac'.
(Other definitions for lens that I've seen before include "Part of photographer's equipment" , "Refracting device" , "feature of camera" , "Camera component" , "Photographer's accessory" .)