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