Camera diaphragm (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">camera diaphragm</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for iris that I've seen before include "pupil's governor" , "may open up in camera" , "--- Murdoch, author" , "who keeps pupil in?" , "Flag,flower and part of the eye" .)