Creature, aptly evolved before Manx cat (8)
I believe the answer is:
platypus
'creature' is the definition.
(platypus is a kind of creature)
'aptly evolved before manx cat' is the wordplay.
'evolved' indicates anagramming the letters (the order of letters changes or evolves).
'before' says to put letters next to each other.
'manx' means to remove the last letter (Manx cats have no tail).
'cat' becomes 'puss' (puss is a kind of cat).
'puss' with its last letter taken off is 'pus'.
'aptly' with letters rearranged gives 'platy'.
'platy'+'pus'='PLATYPUS'
(Other definitions for platypus that I've seen before include "Australian mammal with duckbill" , "Unusual Australian" , "Primitive mammal, duck-billed" , "Australian resident" , "One's billed in flat manner" .)