Mutating mallard duck devours one animal (9)
I believe the answer is:
armadillo
'animal' is the definition.
(I know that armadillo is a type of animal)
'mutating mallard duck devours one' is the wordplay.
'mutating' indicates an anagram (I've seen 'mutated' mean this).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'devours' indicates putting letters inside (inserted letters are devoured).
'one' becomes 'i' (Roman numeral).
'mallard'+'o'='mallardo'
'mallardo' anagrammed gives 'armadllo'.
'armadllo' going around 'i' is 'ARMADILLO'.
(Other definitions for armadillo that I've seen before include "Mammal with protective plates" , "animal on the loose!" , "Molar laid out for the horny-plated animal" , "protected species" , "Mammal covered with bony plates" .)