Lessons conducted in river-beds (7)

I believe the answer is:

courses

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'beds' is the definition.
(I know this)

'lessons conducted in river' is the wordplay.
'lessons' becomes 'course' (a course has lessons).
'conducted in river' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'course'+'s'='COURSES'

Can you help me to learn more?

(Other definitions for courses that I've seen before include "Paths; sequences" , "starters, perhaps" , "Phases of a meal" , "Series of lessons" , "Directions or curricula" .)

I've seen this clue in the Sydney Morning Herald.

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