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" .)
'permission to proceed' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for all clear that I've seen before include "no doubts" , "Understood" , "thumbs up!" , "safety announcement" , "welcome sound" .)