Go-ahead for evacuation (9)
I believe the answer is:
clearance
I believe this clue is a double definition.
'go-ahead' is the first definition.
(I've seen this before)
'for evacuation' is the second definition.
(I've seen this before)
(Other definitions for clearance that I've seen before include "Sanction" , "Authority" , "Official authorisation" , "Removing ball from defensive area" , "Green light" .)