One is underneath black bird (4)
I believe the answer is:
ibis
'bird' is the definition.
(ibis is a kind of bird)
'one is underneath black' is the wordplay.
'one' becomes 'i' (Roman numeral).
'underneath' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'black' becomes 'b' (as in pencil classifications).
'is' after 'b' is 'bis'.
'i'+'bis'='IBIS'
(Other definitions for ibis that I've seen before include "Large wader" , "Eastern" , "Wading bird of warm regions" , "Wading bird with a down-curved bill" , "Bird sacred in Ancient Egypt" .)