Old killer maintains right of returning, in case (9)
I believe the answer is:
portfolio
'case' is the definition.
(I know that portfolio is a type of case)
'old killer maintains right of returning' is the wordplay.
'old killer' becomes 'polio' (I can't explain this - if you can you should give a lot more credence to this answer).
'maintains' is an insertion indicator.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'returning' says the letters should be written backwards.
'of' in reverse letter order is 'fo'.
'rt'+'fo'='rtfo'
'polio' going around 'rtfo' is 'PORTFOLIO'.
'in' is the link.
Can you help me to learn more?
(Other definitions for portfolio that I've seen before include "minister's office" , "A set of investment" , "paper container" , "Set of investments held by one person" , "art collection" .)