Old bone on one antelope (5)
I believe the answer is:
oribi
'antelope' is the definition.
(I've seen this in another clue)
'old bone on one' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bone' becomes 'rib' (rib is a type of bone).
'on' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'o'+'rib'+'i'='ORIBI'
(Other definitions for oribi that I've seen before include "Beastly sort" , "Savannah antelope" , "One grazing" , "Creature" , "Graceful runner" .)