Creature has article to mail off (6)
I believe the answer is:
animal
'creature' is the definition.
(I know that creature can be written as animal)
'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'
'has' acts as a link.
(Other definitions for animal that I've seen before include "Brute - one of the Muppets" , "Beast from Manila" , "Such as 13 ac" , "Brutal, sensual" , "Kind of instinct" .)