Visible horizon (7)
I believe the answer is:
skyline
'visible horizon' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for skyline that I've seen before include "Built-up horizon" , "Silhouette or outline of buildings" , "Horizon; like NY's (anag.)" , "Visible horizon" , "Outline of hills or buildings against firmament" .)