Slips; transparencies (6)
I believe the answer is:
slides
I believe this is a double definition.
'slips' is the first definition.
('slide' can be a synonym of 'slip')
'transparencies' is the second definition.
(slide is a kind of transparency)
(Other definitions for slides that I've seen before include "Sloping chutes in playgrounds" , "Pieces of glass for study of specimens" , "Pieces of glass under the microscope" , "Sloping chutes in the playground" , "Slips, slithers" .)