Hart is wild animal (4)
I believe the answer is:
tahr
'animal' is the definition.
(I've seen this before)
'hart is wild' is the wordplay.
'is wild' is an anagram indicator.
'hart' with letters rearranged gives 'TAHR'.
(Other definitions for tahr that I've seen before include "Himalayan goat" , "goat in Himalayas" , "Muslim ruler; ere me (anag.)" , "animal" .)