Work on bone problem for marsupial (7)
I believe the answer is:
opossum
'marsupial' is the definition.
(opossum is a kind of marsupial)
'work on bone problem' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another.
'bone' becomes 'os' ('os' can be a synonym of 'bone').
'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" , "American animal" , "American arboreal marsupial" , "Thickly furred marsupial" , "American and Australian marsupial, often with the first letter dropped" .)