African veto on leaders of trade union (5)
I believe the answer is:
bantu
'african' is the definition.
(Bantu is an example)
'veto on leaders of trade union' is the wordplay.
'veto' becomes 'ban' (banning is a kind of vetoing).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'leaders of' suggests taking the first letters.
The initial letters of 'trade union' is 'tu'.
'ban'+'tu'='BANTU'
(Other definitions for bantu that I've seen before include "Widely-spoken group of African languages" , "Central and southern African native" , "Native African language" , "Language group including Swahili, Xhosa and Zulu" , "Some Africans" .)