One writer promoted African runner (5)
I believe the answer is:
oribi
'african runner' is the definition.
'oribi' can be an answer for 'runner' (I have seen 'Graceful runner' mean 'oribi' so perhaps 'runner' could also mean 'oribi'). I'm unsure of the 'african' bit.
'one writer promoted' is the wordplay.
'one' becomes 'i' (Roman numeral).
'writer' becomes 'biro' (type of pen, something which writes).
'promoted' says the letters should be written in reverse (going up, in a down clue).
'i'+'biro'='ibiro'
'ibiro' reversed gives 'ORIBI'.
Can you help me to learn more?
(Other definitions for oribi that I've seen before include "Savannah antelope" , "Beastly sort" , "Small African antelope" , "type of antelope" , "One grazing" .)