A horse is not all bad (8)
I believe the answer is:
stallion
'a horse' is the definition.
(I know that stallion is a type of horse)
'is not all bad' is the wordplay.
'bad' indicates an anagram.
'is'+'not'+'all'='isnotall'
'isnotall' anagrammed gives 'STALLION'.
(Other definitions for stallion that I've seen before include "Male horse kept for breeding" , "Stud" , "Male horse lost in atolls" , "Mare's mate" , "Mount" .)