Post Office sent over very large amount resident of Australia carried in pouch (7)
I believe the answer is:
opossum
'resident of australia carried in pouch' is the definition.
I can't tell whether this definition defines the answer.
'post office sent over very large amount' is the wordplay.
'post office' becomes 'po' (abbreviation).
'sent over' says the letters should be written backwards.
'very large' becomes 'os' (abbreviation for oversize).
'amount' becomes 'sum' (synonyms).
'po' back-to-front is 'op'.
'op'+'os'+'sum'='OPOSSUM'
Can you help me to learn more?
(Other definitions for opossum that I've seen before include "Thick-furred marsupial of the Americas" , "American animal" , "Thickly furred marsupial" , "Tree climber" , "Large marsupial" .)