Work a very large amount with an animal (7)
I believe the answer is:
opossum
'an animal' is the definition.
(opossum is a kind of animal)
'work a very large amount' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'a very large' becomes 'os' (abbreviation for oversize).
'amount' becomes 'sum' (synonyms).
'op'+'os'+'sum'='OPOSSUM'
'with' acts as a link.
(Other definitions for opossum that I've seen before include "Type of marsupial" , "Large marsupial" , "American native" , "Small Australian marsupial" , "Small arboreal marsupial" .)