The most common and most likely answer to this crossword clue is the 12 letter word POLYETHYLENE
I've seen this crossword clue in The Times
P
O
L
Y
E
T
H
Y
L
E
N
E
Buddy explains!
The straight part
We know that "Plastic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plastic = polyethylene.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does POLYETHYLENE mean?
POLYETHYLENE noun- A polymer consisting of many ethylene monomers bonded together; used for kitchenware, containers etc.