Creature has article to mail off (6)
I believe the answer is:
animal
'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'
'has' is the 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" .)