Put the plastic cup beside the seat (6)
I believe the answer is:
settee
'seat' is the definition.
(I've seen this before)
'put the plastic cup' is the wordplay.
'put' becomes 'set' (setting is a kind of putting).
'the plastic cup' becomes 'tee' (I can't justify this - if you can you should believe this answer much more).
'set'+'tee'='SETTEE'
'beside the' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for settee that I've seen before include "Sofa, couch" , "Comfortable seat for two or more" , "Chesterfield" , "A long couch without arm-rests" , "Get a seat" .)