Block informal address in French language group (5)
I believe the answer is:
bantu
'language group' is the definition.
(African language group)
'block informal address in french' is the wordplay.
'block' becomes 'ban'.
'informal address in french' becomes 'tu' (informal 'you' in French, as opposed to 'vous').
'ban'+'tu'='BANTU'
(Other definitions for bantu that I've seen before include "Group of widely spoken languages of southern Africa" , "Central and southern African native" , "Native African language" , "Type of language" , "African language family" .)