Nothing like a duck (3)
I believe the answer is:
nil
I believe this is a double definition.
'nothing' is the first definition.
('nil' can be a synonym of 'nothing')
'a duck' is the second definition.
(I've seen this before)
'like' is the link.
(Other definitions for nil that I've seen before include "There's nothing left" , "Lacking value" , "Don't despair - . . . desperandum" , "0" , "duck" .)