Inlay (5)
(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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">inlay</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)