Article accepted by most suitable animal (5)
I believe the answer is:
beast
'animal' is the definition.
(I know that animal is a type of animal)
'article accepted by most suitable' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'accepted by' means one lot of letters goes inside another (inserted letters are accepted).
'most suitable' becomes 'best' (I've seen this in another clue).
'a' going within 'best' is 'BEAST'.
(Other definitions for beast that I've seen before include "Dangerous animal" , "Inhuman person" , "Brute; creature" , "Large animal -- objectionable person" , "Animal; cruel person" .)