WWI battle (5)
I believe the answer is:
somme
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wwi battle</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 somme that I've seen before include "Battle site in France" , "French river known from World War 1" , "runner" , "River of northern France" , "World War One battle region" .)