Alternatively, black ball (3)
I believe the answer is:
orb
'ball' is the definition.
(both can mean a sphere)
'alternatively black' is the wordplay.
'alternatively' becomes 'or'.
'black' becomes 'b' (abbreviation as in pencil classifications).
'or'+'b'='ORB'
(Other definitions for orb that I've seen before include "Ring" , "Sphere for a sovereign" , "Celestial body; sphere" , "Item of insignia" , "Part of royal regalia" .)