Cover seat plus other plastic (9)
I believe the answer is:
upholster
'cover seat' is the definition.
'upholster' can be an answer for 'cover' (I've seen this before). I'm unsure of the 'seat' bit.
'plus other plastic' is the wordplay.
'plastic' indicates an anagram (plastic can mean easily moulded).
'plus'+'other'='plusother'
'plusother' with letters rearranged gives 'UPHOLSTER'.
Can you help me to learn more?
(Other definitions for upholster that I've seen before include "Stuff and cover chairs etc" , "cover chair" , "Furnish with padding" , "Provide furnishings" , "something to do with furniture" .)