Ruminant mammal (4)
'ruminant mammal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for deer that I've seen before include "maybe does" , "The male has antlers" , "Antlered mammal" , "Like a power switch" , "Grazing animal(s)" .)
(Other definitions for goat that I've seen before include "Animal in zodiac" , "lecherous sort" , "beast with sharp horns" , "Billy - nanny" , "nanny, for example" .)
(Other definitions for okapi that I've seen before include "African relative of the giraffe" , "Central Afr. ruminant" , "Ruminant mammal of Africa" , "Wild animal" , "Animal of central Africa" .)