A Scottish river is out (5)
I believe the answer is:
forth
I believe this is a double definition.
'a scottish river' is the first definition.
(I've seen this before)
'out' is the second definition.
(I've seen this before)
'is' acts as a link.
(Other definitions for forth that I've seen before include "(Go) away" , "Onward; the Firth of - -" , "River and estuary in eastern Scotland" , "Out and away -- UK shipping forecast area" , "Onward direction" .)