A mammal (4)
I believe the answer is:
deer
'a mammal' is the definition.
(deer is a kind of mammal)
This is the entire clue.
(Other definitions for deer that I've seen before include "Antlered mammal" , "The male has antlers" , "Antler-bearing animal" , "Hoofed grazing animal" , "Like a power switch" .)