Most ancient ateliers renovated (8)
I believe the answer is:
earliest
'most ancient' is the definition.
(I've seen this in another clue)
'ateliers renovated' is the wordplay.
'renovated' indicates anagramming the letters.
'ateliers' is an anagram of 'EARLIEST'.
(Other definitions for earliest that I've seen before include "With the least delay" , "Ateliers (anag.)" , "First occurring" , "Nearest the beginning in time" , "Furthest back in time" .)