French refusal to accept unknown line in plastic (5)
I believe the answer is:
nylon
'plastic' is the definition.
(thesaurus)
'french refusal to accept unknown line' is the wordplay.
'french refusal' becomes 'non' ('no' in French).
'to accept' means one lot of letters goes inside another.
'unknown' becomes 'y' (algebra).
'line' becomes 'l' (used when specifying particular lines from a poem).
'y'+'l'='yl'
'non' placed around 'yl' is 'NYLON'.
'in' is the link.
(Other definitions for nylon that I've seen before include "Man-made fabric" , "The first synthetic fibre" , "Cover for leg" , "Plastic used for stockings" , "Yarn for stockings" .)