Duck! Unlike ultimately headless horseman (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a kind of duck)
'unlike ultimately headless horseman' is the wordplay.
'ultimately' indicates one should take the final letters.
'headless' means to remove the first letter.
'horseman' becomes 'rider' (I've seen this before).
The final letter of 'unlike' is 'e'.
'rider' with its initial letter taken away is 'ider'.
'e'+'ider'='EIDER'
(Other definitions for eider that I've seen before include "ie Red (anag.)" , "Northern sea duck" , "Downy sea duck" , "Reed I make into a duck" , "Down-producing duck" .)