Bar raised over fifth and sixth of billiard pockets (8)

'pockets' is the definition.
(marsupium is a kind of pocket)

'bar raised over fifth and sixth of billiard' is the wordplay.
'bar' becomes 'Mars' (chocolate bar).
'raised over fifth' becomes 'upi' (I can't explain this - if you can you should believe this answer much more).
'and' means one lot of letters go next to another.
'sixth of billiard' becomes 'a' (6th letter of 'billiard').
'mars'+'upi'+'a'='MARSUPIA'

Can you help me to learn more?