Bird -- is one black on top? (4)
I believe the answer is:
ibis
'bird' is the definition.
(ibis is a kind of bird)
'is one black on top?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'black' becomes 'b' (as in pencil classifications).
'on top?' says to put letters next to each other (down clues only).
'i'+'b'='ib'
'is' after 'ib' is 'IBIS'.
(Other definitions for ibis that I've seen before include "Wading bird with a down-curved bill" , "Large wading bird" , "Wading bird of warm regions" , "Bird sacred in Ancient Egypt" , "Bird worshipped in Ancient Egypt" .)