Speak at length from keep on Scottish river (4,5)
I believe the answer is:
hold forth
'speak at length' is the definition.
(I've seen this before)
'keep on scottish river' is the wordplay.
'keep on' becomes 'hold' (holding is a kind of keeping on).
'scottish river' becomes 'forth' (I've seen this before).
'hold'+'forth'='HOLD FORTH'
'from' acts as a link.
(Other definitions for hold forth that I've seen before include "Show how to" , "Speak at length" , "lecture" , "speak in public?" , "Speak lengthily, maybe preach" .)