Animal, most successful, devours adult (5)
I believe the answer is:
beast
'animal' is the definition.
(I know that animal can be written as beast)
'most successful devours adult' is the wordplay.
'most successful' becomes 'best'.
'devours' means one lot of letters goes inside another (I've seen this in other clues).
'adult' becomes 'a' (abbreviation).
'best' placed around 'a' is 'BEAST'.
(Other definitions for beast that I've seen before include "Bully" , "Wild animal" , "Traditional poet" , "Monster" , "Monstrous person" .)