Instrument Scotsman found in river (5)
I believe the answer is:
piano
'instrument' is the definition.
(piano is a kind of instrument)
'scotsman found in river' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'found in' indicates putting letters inside.
'river' becomes 'Po' (Italian river).
'ian' inserted inside 'po' is 'PIANO'.
(Other definitions for piano that I've seen before include "Clavier" , "Opposite of forte" , "Direction in music (1): relatively softly" , "Stringed instrument - to be played quietly" , "p (mus.)" .)