Outlaw trade union language (5)
I believe the answer is:
bantu
'language' is the definition.
(Bantu is a group of African languages)
'outlaw trade union' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'trade union' becomes 'TU' (abbreviation for trade union).
'ban'+'tu'='BANTU'
(Other definitions for bantu that I've seen before include "Group of widely spoken languages of southern Africa" , "Native African language" , "Zulu, Swahili etc" , "Language group including Swahili, Xhosa and Zulu" , "Group of Niger-Congo languages" .)