Thick-skinned mammal
I believe the answer is:
hippo
'thick-skinned mammal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hippo that I've seen before include "Aquatic mammal" , "Thick-skinned type" , "Large animal" , "African wallower (abbr)" , "One wallows in mud" .)