What's left of plastic I reused (7)
I believe the answer is:
residue
'what's left' is the definition.
(residue means what's left behind)
'plastic i reused' is the wordplay.
'plastic' indicates anagramming the letters (plastic can mean easily moulded).
'i'+'reused'='ireused'
'ireused' anagrammed gives 'RESIDUE'.
'of' acts as a link.
(Other definitions for residue that I've seen before include "Small amount remaining" , "Remainder, leftover" , "What remains of something" , "Matter left behind after other parts have been taken away" , "Something left over" .)