Openness of projectable photo? (12)
I believe the answer is:
transparency
This clue is a double definition.
'openness' is the first definition.
(I've seen this before)
'projectable photo?' is the second definition.
(a transparency is a photo viewed with a projector)
'of' is the link.
(Other definitions for transparency that I've seen before include "openness" , "Film slide" , "Photographic slide" .)