Marsupial work needing very large amount (7)
I believe the answer is:
opossum
'marsupial' is the definition.
(opossum is a kind of marsupial)
'work needing very large amount' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'needing' means one lot of letters go next to another.
'very large' becomes 'os' (abbreviation for oversize).
'amount' becomes 'sum' (synonyms).
'op'+'os'+'sum'='OPOSSUM'
(Other definitions for opossum that I've seen before include "Large marsupial" , "New World marsupial" , "American animal" , "Type of marsupial" , "American and Australian marsupial, often abbreviated" .)