When finished, hide plain belt (6,7)
I believe the answer is:
patent leather
'when finished hide' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'plain belt' is the wordplay.
'plain' becomes 'patent' (synonyms).
'belt' becomes 'leather' (I can't explain this - if you can you should believe this answer much more).
'patent'+'leather'='PATENT LEATHER'
Can you help me to learn more?
(Other definitions for patent leather that I've seen before include "Varnished hide" , "shiny skin" , "it will present a varnished surface" , "Tanned animal skin made hard and glossy" , "Glossy shoe material" .)