Permission to proceed (5)
(Other definitions for leave that I've seen before include "Time off, depart" , "taken by those on the way out" , "Permission (to take holidays?)" , "Soldier's holiday" , "ditch" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">permission to proceed</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 all clear that I've seen before include "no doubts" , "Understood" , "thumbs up!" , "safety announcement" , "welcome sound" .)