Like a mad dog worrying a bird (5)
I believe the answer is:
rabid
'like a mad dog' is the definition.
(I've seen this in another clue)
'worrying a bird' is the wordplay.
'worrying' indicates an anagram.
'a'+'bird'='abird'
'abird' with letters rearranged gives 'RABID'.
(Other definitions for rabid that I've seen before include "Violent; hydrophobic" , "Extremely fanatical" , "Ill from a mad dog bite?" , "Affected by hydrophobia" , "Infected with hydrophobia" .)