Deer have small black sinuous tails (4)
I believe the answer is:
elks
'deer' is the definition.
(I've seen this before)
'have small black sinuous tails' is the wordplay.
'tails' indicates one should take the final letters (I've seen 'tail' mean this).
The final letters of 'have small black sinuous' are 'ELKS'.
(Other definitions for elks that I've seen before include "Large deer with flattened antlers" , "Moose" , "Moose(s)" , "Large deer of Northern Europe" , "Large deer of Europe and N America" .)