Ass over-protects horse (6)
I believe the answer is:
onager
'ass' is the definition.
(onager is a kind of ass)
'over-protects horse' is the wordplay.
'over' becomes 'oer' (synonyms).
'protects' indicates putting letters inside.
'horse' becomes 'nag' (term for an old horse).
'oer' enclosing 'nag' is 'ONAGER'.
(Other definitions for onager that I've seen before include "Variety of wild ass found in central Asia" , "Strangely, the wild ass might be orange" , "The ass is a strange orange" , "donkey" , "wild beast" .)