Work problem involves big animal (7)
I believe the answer is:
opossum
'animal' is the definition.
(opossum is a kind of animal)
'work problem involves big' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'problem' becomes 'sum' (as in a mathematical problem).
'involves' indicates putting letters inside.
'big' becomes 'os' (abbreviation for oversize).
'op'+'sum'='opsum'
'opsum' placed around 'os' is 'OPOSSUM'.
(Other definitions for opossum that I've seen before include "Native of America" , "New World marsupial" , "American tree-dweller" , "American and Australian marsupial, often with the first letter dropped" , "Small Australian marsupial" .)