Marsupial is round, churning moss up (7)
I believe the answer is:
opossum
'marsupial' is the definition.
(I know that opossum is a type of marsupial)
'round churning moss up' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'churning' indicates an anagram.
'moss'+'up'='mossup'
'mossup' is an anagram of 'possum'.
'o'+'possum'='OPOSSUM'
'is' acts as a link.
(Other definitions for opossum that I've seen before include "Tree climber" , "US marsupial" , "Thick-furred marsupial of the Americas" , "American arboreal marsupial" , "American marsupial with prehensile tail" .)