Marsupial's drunk some unfinished soup (7)
I believe the answer is:
opossum
'marsupial's' is the definition.
(opossum is a kind of marsupial)
'drunk some unfinished soup' is the wordplay.
'drunk' indicates anagramming the letters.
'unfinished' means to remove the last letter.
'some' with its last letter taken away is 'som'.
'som'+'soup'='somsoup'
'somsoup' is an anagram of 'OPOSSUM'.
(Other definitions for opossum that I've seen before include "American and Australian marsupial, often with the first letter dropped" , "Type of marsupial" , "American tree-dweller" , "US marsupial" , "New World marsupial" .)