Farmer once in rising river amusing duck (9)

'farmer once' is the definition.
(Old Macdonald)

'rising river amusing duck' is the wordplay.
'rising' says the letters should be written backwards (going up in a down clue).
'river' becomes 'Cam' (river in Cambridge, England).
'amusing duck' becomes 'Donald' (Disney character Donald Duck).
'cam' reversed gives 'mac'.
'mac'+'donald'='MACDONALD'

'in' is the link.