Diaphragm that controls the size of the pupil (4)
I believe the answer is:
iris
'diaphragm that controls the size of the pupil' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iris that I've seen before include "Pupil is at the centre of this" , "One controlling pupil' s reactions" , "Flag - part of eye" , "Fleur-de-lis" , "Rainbow-goddess" .)