Crow's-nest site
I believe the answer is:
mast
'crow's-nest site' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mast that I've seen before include "Pole supporting ship's sails" , "Nuts for pigs" , "Long upright pole that supports sails" , "pig-food" , "Main vertical spar of vessel" .)