Permission to proceed -- there's headroom enough (9)
I believe the answer is:
clearance
This clue is a double definition.
'permission to proceed' is the first definition.
(I've seen this before)
'headroom enough' is the second definition.
(as in headroom for a vehicle)
'there's' acts as a link.
(Other definitions for clearance that I've seen before include "Eviction from lands" , "Official permission (for an action)" , "Headroom" , "Vacation" , "eviction from land" .)