Ahead of time, sets out part of mosaic (7)
I believe the answer is:
tessera
'part of mosaic' is the definition.
(I've seen this before)
'ahead of time sets out' is the wordplay.
'ahead of' is a charade indicator (letters next to each other) (some letters go in front of others).
'time' becomes 'era' (an era is a period of time).
'out' indicates an anagram (out can mean wrong or inaccurate).
'sets' with letters rearranged gives 'tess'.
'era' after 'tess' is 'TESSERA'.
(Other definitions for tessera that I've seen before include "Bit of design" , "Small stone for mosaics" , "Bit of mosaic" , "Mosaic component" , "Reseats (anag) -- small tile used in a mosaic" .)