Extra, in reserve (5)
I believe the answer is:
spare
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">extra</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(both can mean left over)</span><br/ ><br/ >'<span class="explanation-format__original">in reserve</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for spare that I've seen before include "In reserve; pardon" , "Let off; extra" , "show mercy" , "Surplus to requirements" , "Don't use" .)