Creature ailing badly (6)
I believe the answer is:
nilgai
'creature' is the definition.
(I've seen this before)
'ailing badly' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'ailing' is an anagram of 'NILGAI'.
(Other definitions for nilgai that I've seen before include "Large Indian antelope, ailing (anag.)" , "animal" , "creature" .)