Work price includes huge marsupial (7)

I believe the answer is:

opossum

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'marsupial' is the definition.
(I know that opossum is a type of marsupial)

'work price includes huge' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'price' becomes 'sum' (I can't explain this - if you can you should believe this answer much more).
'includes' indicates putting letters inside.
'huge' becomes 'os' (abbreviation for outsize).
'op'+'sum'='opsum'
'opsum' placed around 'os' is 'OPOSSUM'.

Can you help me to learn more?

(Other definitions for opossum that I've seen before include "Tree climber" , "Small Australian marsupial" , "American marsupial with prehensile tail" , "Small arboreal marsupial" , "Thick-furred marsupial of the Americas" .)

I've seen this clue in The Daily Express.

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