Python and eel writhing in plastic (9)
I believe the answer is:
polythene
'plastic' is the definition.
(I've seen this before)
'python and eel writhing' is the wordplay.
'and' says to put letters next to each other.
'writhing' indicates an anagram.
'python'+'eel'='pythoneel'
'pythoneel' anagrammed gives 'POLYTHENE'.
'in' is the link.
(Other definitions for polythene that I've seen before include "Strange telephony provides a lightweight material, used in packaging" , "Synthetic resin" , "Synthetic 6 Dn" , "A light plastic material used for wrapping" , "Synthetic thermoplastic used for packaging, etc" .)