Second row about Kentucky’s visible horizon (7)
I believe the answer is:
skyline
'visible horizon' is the definition.
(synonyms)
'second row about kentucky's' is the wordplay.
'second' becomes 's' (common abbreviation).
'row' becomes 'line' (as in a row of things).
'about' means one lot of letters go next to another.
'kentucky' becomes 'ky' (abbreviation).
'line' put after 'ky' is 'kyline'.
's'+'kyline'='SKYLINE'
(Other definitions for skyline that I've seen before include "Horizon; like NY's (anag.)" , "Outline of hills or buildings against firmament" , "Silhouette or outline of buildings" , "City horizon" , "Built-up horizon" .)