Hoofed mammal (5)
(Other definitions for horse that I've seen before include "This common sense is of equine origin" , "Animal with withers" , "Nag, hack" , "punch, perhaps" , "Ridden animal" .)
'hoofed mammal' is the definition.
(hippo is a kind of hoofed mammal)
This is the entire clue.
(Other definitions for hippo that I've seen before include "Large semi-aquatic mammal (abbr)" , "Wallowing creature" , "Thick-skinned type" , "Semiaquatic animal" , "one that's large and fleshy" .)