Margin of safety (6)
I believe the answer is:
leeway
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">margin of safety</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 leeway that I've seen before include "Permissible difference, allowing some freedom to move" , "play" , "Amount of freedom" , "Margin of tolerance" , "Drift to make up" .)