Furthest extent (5)
I believe the answer is:
limit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">furthest extent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for limit that I've seen before include "verge" , "Restrict - boundary" , "Outer extent" , "Put restriction on" , "maximum number" .)