Beast has article to mail off (6)
I believe the answer is:
animal
'beast' is the definition.
(a beast is an 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 "savage" , "Living creature with voluntary motion" , "One sharing our kingdom" , "Beast from Manila" , "Neither vegetable nor mineral" .)