What's left over (8)
I believe the answer is:
residual
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">what's left over</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for residual that I've seen before include "Leftover - used rail (anag)" , "Concerning what is left over" , "Remaining at the end" , "'Remaining, left over (8)'" , "Remaining; repeat royalty" .)