Light thermoplastic (9)

I believe the answer is:

polythene

(Other definitions for polythene that I've seen before include "Synthetic thermoplastic used for packaging, etc" , "Synthetic 6 Dn" , "Synthetic resin" , "Strange telephony provides a lightweight material, used in packaging" , "A lightweight thermoplastic" .)

I've seen this clue in The Sunday Times and The Times.

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