Large, Indian antelope is sadly ailing (6)
I believe the answer is:
nilgai
'large indian antelope' is the definition.
(I've seen this in another clue)
'sadly ailing' is the wordplay.
'sadly' indicates anagramming the letters.
'ailing' is an anagram of 'NILGAI'.
'is' acts as a link.
(Other definitions for nilgai that I've seen before include "Large Indian antelope, ailing (anag.)" , "creature" , "animal" .)