Diaphragm that controls the size of the pupil (4)
I believe the answer is:
iris
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">diaphragm that controls the size of the pupil</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)