A donkey, virtually wild (4)
I believe the answer is:
amok
'wild' is the definition.
(eg running amok is running wild)
'a donkey virtually' is the wordplay.
'donkey' becomes 'moke' (moke can informally mean a donkey).
'virtually' means to remove the last letter (all but the last letter).
'moke' with its last letter removed is 'mok'.
'a'+'mok'='AMOK'
(Other definitions for amok that I've seen before include "crazy" , "Berserk" , "In an uncontrolled, often violent, way" , "(Run) in murderous frenzy" , "Wild and uncontrollable" .)