Plastic bag finally forsook for good (best) (8)
I believe the answer is:
bakelite
'plastic' is the definition.
(Bakelite is an example)
'good best' is the wordplay.
'good' becomes 'bak' (I can't justify this - if you can you should give a lot more credence to this answer).
'best' becomes 'elite' (I've seen this before).
'bak'+'elite'='BAKELITE'
'bag finally forsook for' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for bakelite that I've seen before include "Type of hard plastic, in old telephones say" , "Early form of plastic used for tableware, telephones etc" , "'Type of hard plastic, used in old telephones (8)'" .)