Article to mail off to creature (6)
I believe the answer is:
animal
'to creature' is the definition.
(synonyms)
'article to mail off' is the wordplay.
'article' becomes 'an' (indefinite article).
'off' indicates anagramming the letters.
'mail' with letters rearranged gives 'imal'.
'an'+'imal'='ANIMAL'
(Other definitions for animal that I've seen before include "Creature having voluntary movement" , "Any organism capable of voluntary movement" , "Source of meat" , "Such as 13 ac" , "Neither vegetable nor mineral" .)