She opens up when it's dark (4)
'she' is the definition.
(I've seen this before)
'opens up when it's dark' is the wordplay.
I cannot quite see how this works, but
'up' could be 'is' (I've seen this in other clues) and 'is' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for iris that I've seen before include "Pupil is at the centre of this", "Woman", "Fleur-de-lys", "- - Murdoch, novelist", "may open up in camera".)