Some polypropylene is of poor quality (4)
I believe the answer is:
ropy
'of poor quality' is the definition.
(I've seen this before)
'some polypropylene is' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ROPY' is hidden in the letters of 'polypropylene is'.
(Other definitions for ropy that I've seen before include "Of poor quality (informal)" , "like some painters?" , "Not very good" , "Of dubious quality" , "Slightly off-colour" .)