Work on huge problem for marsupial (7)
I believe the answer is:
opossum
'marsupial' is the definition.
(opossum is a kind of marsupial)
'work on huge problem' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another.
'huge' becomes 'os' (abbreviation for outsize).
'problem' becomes 'sum' (arithmetic problem).
'op'+'os'+'sum'='OPOSSUM'
'for' acts as a link.
(Other definitions for opossum that I've seen before include "Small arboreal marsupial" , "Thick-furred marsupial of the Americas" , "American arboreal marsupial" , "American animal" , "American and Australian marsupial, often abbreviated" .)