Polish round seed case
I believe the answer is:
bur
'seed case' is the definition.
'bur' can be an answer for 'seed' (I have seen 'Prickly seed ' mean 'bur' so perhaps 'seed' could also mean 'bur'). I'm unsure of the 'case' bit.
'polish round' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'round' is a reversal indicator.
'rub' written backwards gives 'BUR'.
Can you help me to learn more?
(Other definitions for bur that I've seen before include "Clinging seed case" , "Seed-case" , "sticker" , "Prickly seed case" , "small drill" .)