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