Most primitive ateliers badly built (8)
I believe the answer is:
earliest
'most primitive' is the definition.
('early' can be similar in meaning to 'primitive')
'ateliers badly built' is the wordplay.
'ateliers' can be anagrammed to 'EARLIEST'.
Though, I'm not clear how this is indicated.
Can you help me to learn more?
(Other definitions for earliest that I've seen before include "With the least delay" , "First occurring" , "Arriving first" , "Original or most previous" , "Furthest back in time" .)