Vote backing a dominant African (5)
I believe the answer is:
xhosa
'african' is the definition.
(the Xhosa are an African ethnic group)
'vote backing a dominant' is the wordplay.
'vote' becomes 'x' (how votes are marked).
'backing' is a reversal indicator.
'dominant' becomes 'soh' (dominant musical note in the do-re-mi system).
'a'+'soh'='asoh'
'asoh' reversed gives 'hosa'.
'x'+'hosa'='XHOSA'
(Other definitions for xhosa that I've seen before include "Clicking tongue" , "Tribesman" , "South African language spoken with clicks" , "Cattle-rearer in South Africa" , "South African tribe" .)