Animal that may be ailing (6)
I believe the answer is:
nilgai
'animal' is the definition.
(I've seen this before)
'may be ailing' is the wordplay.
'may be' is an anagram indicator.
'ailing' anagrammed gives 'NILGAI'.
'that' acts as a link.
(Other definitions for nilgai that I've seen before include "Large Indian antelope, ailing (anag.)" , "animal" , "creature" .)