A year on river somewhere in Scotland (3)
I believe the answer is:
ayr
'somewhere in scotland' is the definition.
(Scottish town)
'a year on river' is the wordplay.
'year' becomes 'y'.
'on' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'a'+'y'+'r'='AYR'
(Other definitions for ayr that I've seen before include "Scottish seaport" , "Scottish city" , "Scots port on Firth of Clyde" , "Scottish town, port and racecourse" , "Race course" .)