Most former ateliers rebuilt (8)
I believe the answer is:
earliest
'most former' is the definition.
('early' can be a synonym of 'former')
'ateliers rebuilt' is the wordplay.
'rebuilt' indicates an anagram.
'ateliers' is an anagram of 'EARLIEST'.
(Other definitions for earliest that I've seen before include "The first to arrive - to get the worm?" , "First occurring" , "Ateliers (anag.)" , "Nearest the start of a time period" , "Next available" .)