Plastic slide displayed by top class gallery (7)
I believe the answer is:
acetate
'plastic' is the definition.
(thesaurus)
'slide displayed by top class gallery' is the wordplay.
'slide' becomes 'ce' (I can't explain this - if you can you should believe this answer much more).
'displayed by' means one lot of letters go next to another.
'top class' becomes 'a' (top grade).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'ce' put after 'a' is 'ace'.
'ace'+'tate'='ACETATE'
Can you help me to learn more?
(Other definitions for acetate that I've seen before include "It once covered records" , "film on overhead projector" , "Cellulose ester used in fabrics, LPs etc." , "Textile fibre, plastic" , "Material in old record" .)