Inlay (6)
(Other definitions for mosaic that I've seen before include "Picture made with pieces of glass and stone" , "work of art" , "Tessellated picture" , "Artistic design of glass or stones" , "Pattern made with tiny pieces of stone or glass" .)
'inlay' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for inset that I've seen before include "one image within another" , "day for teachers" , "Thing inserted" , "Part of picture" , "Not taking proper care" .)