Old pro with large amount to obtain a marsupial (7)
I believe the answer is:
opossum
'a marsupial' is the definition.
(opossum is a kind of marsupial)
'old pro with large amount' is the wordplay.
'old pro' becomes 'op' (this could be a standard abbreviation I've not previously seen).
'with' means one lot of letters go next to another.
'large' becomes 'os'.
'amount' becomes 'sum' (synonyms).
'op'+'os'+'sum'='OPOSSUM'
'to obtain' acts as a link.
Can you help me to learn more?
(Other definitions for opossum that I've seen before include "Tree climber" , "US marsupial" , "American native" , "Small Australian marsupial" , "Small arboreal marsupial" .)