Quiet Scotsman seen in river (5)
I believe the answer is:
piano
'quiet' is the definition.
(I've seen this before)
'scotsman seen in river' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'seen in' indicates putting letters inside.
'river' becomes 'Po River' (Italian river).
'ian' inserted within 'po' is 'PIANO'.
(Other definitions for piano that I've seen before include "Instrument with ivories" , "p (mus.)" , "one of 6 [STEINWAYS]" , "Soft, softly (music)" , "Clavier" .)