Type of marsupial - work with very large amount (7)
I believe the answer is:
opossum
'type of marsupial' is the definition.
(I've seen this before)
'work with very large amount' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'with' is a charade indicator (letters next to each other).
'very large' becomes 'os' (abbreviation for oversize).
'amount' becomes 'sum' ('sum' can be a synonym of 'amount').
'op'+'os'+'sum'='OPOSSUM'
(Other definitions for opossum that I've seen before include "Large marsupial" , "American native" , "American arboreal marsupial" , "American tree-dweller" , "US marsupial" .)