Mountaintop nest (5)
I believe the answer is:
aerie
'mountaintop nest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aerie that I've seen before include "Visionary" , "brood" , "Lofty nest (alternative spelling)" , "wherein some types of bird may sit" , "inaccessible place" .)