Formal permission (5)
'formal permission' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for leave that I've seen before include "Permission to do something" , "Time off; go" , "Skip" , "Insufficient foliage" , "Don't touch" .)
(Other definitions for licence that I've seen before include "Legal document giving official permission" , "Official authorisation" , "Permit - freedom" , "Authority (or the lack of it!)" , "Authority - or freedom!" .)