Knockout celebration doesn't start for marsupial (5)
I believe the answer is:
koala
'for marsupial' is the definition.
(koala is a kind of marsupial)
'knockout celebration doesn't start' is the wordplay.
'knockout' becomes 'KO' (abbreviation).
'celebration' becomes 'gala' (gala is a kind of celebration).
'doesn't start' means to remove the first letter.
'gala' with its initial letter removed is 'ala'.
'ko'+'ala'='KOALA'
(Other definitions for koala that I've seen before include "Eater of eucalyptus" , "leaf-eater" , "small bear" , "Australian tree-climbing marsupial with grey fur and big ears" , "Aussie tree-climber" .)