Crossword Genius

Speak at length from keep on Scottish river (4,5)

Ross

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" .)

I've seen this clue in The Telegraph.
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