Supplement by adding clear finish to popular building material (9)
I believe the answer is:
increment
'material' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'supplement by adding clear finish to popular' is the wordplay.
'supplement' becomes 'cement' (I can't explain this - if you can you should give a lot more credence to this answer).
'by adding' indicates putting letters inside.
'finish' indicates one should take the final letters.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'popular' becomes 'in' ('in' can mean trendy).
The final letter of 'clear' is 'r'.
'cement' enclosing 'r' is 'crement'.
'crement' put after 'in' is 'INCREMENT'.
'building' acts as a link.
Can you help me to learn more?
(Other definitions for increment that I've seen before include "growth" , "Small change" , "Rise" , "Step" , "Addition to wages etc" .)