Copper frames left by kind of white, synthetic substance (7)
I believe the answer is:
plastic
'synthetic substance' is the definition.
(I've seen this before)
'copper frames left by kind of white' is the wordplay.
'copper' becomes 'PC' (police constable).
'frames' indicates putting letters inside (outside letters frame the inside letters).
'left by' becomes 'l' (I've seen this in other clues).
'kind of white' becomes 'asti' (I can't justify this - if you can you should give a lot more credence to this answer).
'l'+'asti'='lasti'
'pc' placed around 'lasti' is 'PLASTIC'.
Can you help me to learn more?
(Other definitions for plastic that I've seen before include "Scalp it badly to make a synthetic material" , "non-metallic material" , "(Originally malleable) material" , "Credit card; mouldable" , "Artificial/synthetic" .)