Approval of evacuation (9)
I believe the answer is:
clearance
I believe this is a double definition.
'approval' is the first definition.
(thesaurus)
'evacuation' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for clearance that I've seen before include "Authority" , "spare room" , "Official permission (for an action)" , "Green light" , "Headroom" .)