Map addition (5)
I believe the answer is:
inset
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">map addition</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for inset that I've seen before include "Something inserted" , "subsidiary diagram" , "Put (one thing) into (another)" , "Small picture placed within a larger one" , "Implant" .)